Re: [pve-devel] [PATCH qemu-server v4 3/4] config: add AMD SEV-SNP support.

[Markus Frank <m.frank@proxmox.com>]

On  2025-03-11 16:04, Philipp Giersfeld wrote:
> This patch is for enabling AMD SEV-SNP support.
> 
> Where applicable, it extends support for existing SEV(-ES) variables
> to SEV-SNP. This means that it retains no-debug and kernel-hashes
> options, but the no-key-sharing option is removed.
> 
> The default policy value is identical to QEMU’s, and the therefore
> required option has been added to configure SMT support.
> 
> The code was tested by running a VM without SEV, with SEV, SEV-ES,
> SEV-SNP. Each configuration was tested with and without an EFI disk
> attached. For SEV-enabled configurations it was also verified that the
> kernel actually used the respective feature.
> 
> Signed-off-by: Philipp Giersfeld <philipp.giersfeld@canarybit.eu>
> Reviewed-by: Daniel Kral <d.kral@proxmox.com>
> Tested-by: Markus Frank <m.frank@proxmox.com>
I have not tested this version before (there have been some changes since v3/v2).
But now I have. So the "Tested-by" can stay there.
> ---
> 
>   changes since v3: https://lists.proxmox.com/pipermail/pve-devel/2025-February/068578.html
>   * Add and clarify logging statements and descriptions
>   * Add additional safeguards
> 
>   PVE/API2/Qemu.pm            |  7 +++-
>   PVE/QemuServer.pm           | 52 +++++++++++++++++++++--------
>   PVE/QemuServer/CPUConfig.pm | 66 ++++++++++++++++++++++++++++---------
>   3 files changed, 95 insertions(+), 30 deletions(-)
> 
> diff --git a/PVE/API2/Qemu.pm b/PVE/API2/Qemu.pm
> index 295260e7..ef3aa8d6 100644
> --- a/PVE/API2/Qemu.pm
> +++ b/PVE/API2/Qemu.pm
> @@ -548,8 +548,13 @@ my sub create_disks : prototype($$$$$$$$$$$) {
>   		my $volid;
>   		if ($ds eq 'efidisk0') {
>   		    my $smm = PVE::QemuServer::Machine::machine_type_is_q35($conf);
> +
> +		    my $amd_sev_type = PVE::QemuServer::CPUConfig::get_amd_sev_type($conf);
> +		    die "SEV-SNP uses consolidated read-only firmware and does not require an EFI disk\n"
> +			if $amd_sev_type && $amd_sev_type eq 'snp';
> +
>   		    ($volid, $size) = PVE::QemuServer::create_efidisk(
> -			$storecfg, $storeid, $vmid, $fmt, $arch, $disk, $smm);
> +			$storecfg, $storeid, $vmid, $fmt, $arch, $disk, $smm, $amd_sev_type);
>   		} elsif ($ds eq 'tpmstate0') {
>   		    # swtpm can only use raw volumes, and uses a fixed size
>   		    $size = PVE::Tools::convert_size(PVE::QemuServer::Drive::TPMSTATE_DISK_SIZE, 'b' => 'kb');
> diff --git a/PVE/QemuServer.pm b/PVE/QemuServer.pm
> index 808c0e1c..727aad6c 100644
> --- a/PVE/QemuServer.pm
> +++ b/PVE/QemuServer.pm
> @@ -52,7 +52,7 @@ use PVE::QemuConfig;
>   use PVE::QemuServer::Helpers qw(config_aware_timeout min_version kvm_user_version windows_version);
>   use PVE::QemuServer::Cloudinit;
>   use PVE::QemuServer::CGroup;
> -use PVE::QemuServer::CPUConfig qw(print_cpu_device get_cpu_options get_cpu_bitness is_native_arch get_amd_sev_object);
> +use PVE::QemuServer::CPUConfig qw(print_cpu_device get_cpu_options get_cpu_bitness is_native_arch get_amd_sev_object get_amd_sev_type);
>   use PVE::QemuServer::Drive qw(is_valid_drivename checked_volume_format drive_is_cloudinit drive_is_cdrom drive_is_read_only parse_drive print_drive);
>   use PVE::QemuServer::Machine;
>   use PVE::QemuServer::Memory qw(get_current_memory);
> @@ -88,6 +88,13 @@ my $OVMF = {
>   	    "$EDK2_FW_BASE/OVMF_CODE_4M.secboot.fd",
>   	    "$EDK2_FW_BASE/OVMF_VARS_4M.ms.fd",
>   	],
> +	'4m-sev' => [
> +	    "$EDK2_FW_BASE/OVMF_CVM_CODE_4M.fd",
> +	    "$EDK2_FW_BASE/OVMF_CVM_VARS_4M.fd",
> +	],
> +	'4m-snp' => [
> +	    "$EDK2_FW_BASE/OVMF_CVM_4M.fd",
> +	],
>   	# FIXME: These are legacy 2MB-sized images that modern OVMF doesn't supports to build
>   	# anymore. how can we deperacate this sanely without breaking existing instances, or using
>   	# older backups and snapshot?
> @@ -3172,15 +3179,22 @@ sub vga_conf_has_spice {
>       return $1 || 1;
>   }
>   
> -sub get_ovmf_files($$$) {
> -    my ($arch, $efidisk, $smm) = @_;
> +sub get_ovmf_files($$$$) {
> +    my ($arch, $efidisk, $smm, $amd_sev_type) = @_;
>   
>       my $types = $OVMF->{$arch}
>   	or die "no OVMF images known for architecture '$arch'\n";
>   
>       my $type = 'default';
>       if ($arch eq 'x86_64') {
> -	if (defined($efidisk->{efitype}) && $efidisk->{efitype} eq '4m') {
> +	if ($amd_sev_type && $amd_sev_type eq 'snp') {
> +	    $type = "4m-snp";
> +	    my ($ovmf) = $types->{$type}->@*;
> +	    die "EFI base image '$ovmf' not found\n" if ! -f $ovmf;
> +	    return ($ovmf);
> +	} elsif ($amd_sev_type) {
> +	    $type = "4m-sev";
> +	} elsif (defined($efidisk->{efitype}) && $efidisk->{efitype} eq '4m') {
>   	    $type = $smm ? "4m" : "4m-no-smm";
>   	    $type .= '-ms' if $efidisk->{'pre-enrolled-keys'};
>   	} else {
> @@ -3329,7 +3343,10 @@ my sub print_ovmf_drive_commandlines {
>   
>       my $d = $conf->{efidisk0} ? parse_drive('efidisk0', $conf->{efidisk0}) : undef;
>   
> -    my ($ovmf_code, $ovmf_vars) = get_ovmf_files($arch, $d, $q35);
> +    my $amd_sev_type = get_amd_sev_type($conf);
> +    die "Attempting to configure SEV-SNP with flash devices instead of using `-bios`\n"
> +	if $amd_sev_type && $amd_sev_type eq 'snp';
> +    my ($ovmf_code, $ovmf_vars) = get_ovmf_files($arch, $d, $q35, $amd_sev_type);
>   
>       my $var_drive_str = "if=pflash,unit=1,id=drive-efidisk0";
>       if ($d) {
> @@ -3526,10 +3543,18 @@ sub config_to_command {
>   	die "OVMF (UEFI) BIOS is not supported on 32-bit CPU types\n"
>   	    if !$forcecpu && get_cpu_bitness($conf->{cpu}, $arch) == 32;
>   
> -	my ($code_drive_str, $var_drive_str) =
> -	    print_ovmf_drive_commandlines($conf, $storecfg, $vmid, $arch, $q35, $version_guard);
> -	push $cmd->@*, '-drive', $code_drive_str;
> -	push $cmd->@*, '-drive', $var_drive_str;
> +	my $amd_sev_type = get_amd_sev_type($conf);
> +	if ($amd_sev_type && $amd_sev_type eq 'snp') {
> +	   my $arch = PVE::QemuServer::Helpers::get_vm_arch($conf);
> +	   print "Existing EFI disk will be ignored for SEV-SNP\n"
> +		if parse_drive('efidisk0', $conf->{efidisk0});
> +	   push $cmd->@*, '-bios', get_ovmf_files($arch, undef, undef, $amd_sev_type);
> +	} else {
> +	    my ($code_drive_str, $var_drive_str) = print_ovmf_drive_commandlines(
> +		$conf, $storecfg, $vmid, $arch, $q35, $version_guard);
> +	    push $cmd->@*, '-drive', $code_drive_str;
> +	    push $cmd->@*, '-drive', $var_drive_str;
> +	}
>       }
>   
>       if ($q35) { # tell QEMU to load q35 config early
> @@ -8337,7 +8362,8 @@ sub get_efivars_size {
>       my $arch = PVE::QemuServer::Helpers::get_vm_arch($conf);
>       $efidisk //= $conf->{efidisk0} ? parse_drive('efidisk0', $conf->{efidisk0}) : undef;
>       my $smm = PVE::QemuServer::Machine::machine_type_is_q35($conf);
> -    my (undef, $ovmf_vars) = get_ovmf_files($arch, $efidisk, $smm);
> +    my $amd_sev_type = get_amd_sev_type($conf);
> +    my (undef, $ovmf_vars) = get_ovmf_files($arch, $efidisk, $smm, $amd_sev_type);
>       return -s $ovmf_vars;
>   }
>   
> @@ -8361,10 +8387,10 @@ sub update_tpmstate_size {
>       $conf->{tpmstate0} = print_drive($disk);
>   }
>   
> -sub create_efidisk($$$$$$$) {
> -    my ($storecfg, $storeid, $vmid, $fmt, $arch, $efidisk, $smm) = @_;
> +sub create_efidisk($$$$$$$$) {
> +    my ($storecfg, $storeid, $vmid, $fmt, $arch, $efidisk, $smm, $amd_sev_type) = @_;
>   
> -    my (undef, $ovmf_vars) = get_ovmf_files($arch, $efidisk, $smm);
> +    my (undef, $ovmf_vars) = get_ovmf_files($arch, $efidisk, $smm, $amd_sev_type);
>   
>       my $vars_size_b = -s $ovmf_vars;
>       my $vars_size = PVE::Tools::convert_size($vars_size_b, 'b' => 'kb');
> diff --git a/PVE/QemuServer/CPUConfig.pm b/PVE/QemuServer/CPUConfig.pm
> index ad0be16e..9ae1996b 100644
> --- a/PVE/QemuServer/CPUConfig.pm
> +++ b/PVE/QemuServer/CPUConfig.pm
> @@ -18,6 +18,7 @@ get_cpu_options
>   get_cpu_bitness
>   is_native_arch
>   get_amd_sev_object
> +get_amd_sev_type
>   );
>   
>   # under certain race-conditions, this module might be loaded before pve-cluster
> @@ -231,25 +232,32 @@ my $cpu_fmt = {
>   my $sev_fmt = {
>       type => {
>   	description => "Enable standard SEV with type='std' or enable"
> -	    ." experimental SEV-ES with the 'es' option.",
> +	    ." experimental SEV-ES with the 'es' option or enable"
> +	    ." experimental SEV-SNP with the 'snp' option.",
>   	type => 'string',
>   	default_key => 1,
>   	format_description => "sev-type",
> -	enum => ['std', 'es'],
> +	enum => ['std', 'es', 'snp'],
>   	maxLength => 3,
>       },
>       'no-debug' => {
> -	description => "Sets policy bit 0 to 1 to disallow debugging of guest",
> +	description => "Sets policy bit to disallow debugging of guest",
>   	type => 'boolean',
>   	default => 0,
>   	optional => 1,
>       },
>       'no-key-sharing' => {
> -	description => "Sets policy bit 1 to 1 to disallow key sharing with other guests",
> +	description => "Sets policy bit to disallow key sharing with other guests (Ignored for SEV-SNP)",
>   	type => 'boolean',
>   	default => 0,
>   	optional => 1,
>       },
> +    'allow-smt' => {
> +	description => "Sets policy bit to allow Simultaneous Multi Threading (SMT) (Ignored unless for SEV-SNP)",
> +	type => 'boolean',
> +	default => 1,
> +	optional => 1,
> +    },
>       "kernel-hashes" => {
>   	description => "Add kernel hashes to guest firmware for measured linux kernel launch",
>   	type => 'boolean',
> @@ -823,6 +831,13 @@ sub get_hw_capabilities {
>       }
>       return $hw_capabilities;
>   }
> +sub get_amd_sev_type {
> +    my ($conf) = @_;
> +
> +    return undef if !$conf->{'amd-sev'};
> +
> +    return PVE::JSONSchema::parse_property_string($sev_fmt, $conf->{'amd-sev'})->{type};
> +}
>   
>   sub get_amd_sev_object {
>       my ($amd_sev, $bios) = @_;
> @@ -836,22 +851,41 @@ sub get_amd_sev_object {
>       if ($amd_sev_conf->{type} eq 'es' && !$sev_hw_caps->{'sev-support-es'}) {
>   	die "Your CPU does not support AMD SEV-ES.\n";
>       }
> +    if ($amd_sev_conf->{type} eq 'snp' && !$sev_hw_caps->{'sev-support-snp'}) {
> +	die "Your CPU does not support AMD SEV-SNP.\n";
> +    }
>       if (!$bios || $bios ne 'ovmf') {
>   	die "To use AMD SEV, you need to change the BIOS to OVMF.\n";
>       }
>   
> -    my $sev_mem_object = 'sev-guest,id=sev0';
> -    $sev_mem_object .= ',cbitpos='.$sev_hw_caps->{cbitpos};
> -    $sev_mem_object .= ',reduced-phys-bits='.$sev_hw_caps->{'reduced-phys-bits'};
> -
> -    # guest policy bit calculation as described here:
> -    # https://documentation.suse.com/sles/15-SP5/html/SLES-amd-sev/article-amd-sev.html#table-guestpolicy
> -    my $policy = 0;
> -    $policy |= 1 << 0 if $amd_sev_conf->{'no-debug'};
> -    $policy |= 1 << 1 if $amd_sev_conf->{'no-key-sharing'};
> -    $policy |= 1 << 2 if $amd_sev_conf->{type} eq 'es';
> -    # disable migration with bit 3 nosend to prevent amd-sev-migration-attack
> -    $policy |= 1 << 3;
> +    my $sev_mem_object = '';
> +    my $policy;
> +    if ($amd_sev_conf->{type} eq 'es' || $amd_sev_conf->{type} eq 'std') {
> +	$sev_mem_object .= 'sev-guest,id=sev0';
> +	$sev_mem_object .= ',cbitpos='.$sev_hw_caps->{cbitpos};
> +	$sev_mem_object .= ',reduced-phys-bits='.$sev_hw_caps->{'reduced-phys-bits'};
> +
> +	# guest policy bit calculation as described here:
> +	# https://documentation.suse.com/sles/15-SP5/html/SLES-amd-sev/article-amd-sev.html#table-guestpolicy
> +	$policy = 0;
> +	$policy |= 1 << 0 if $amd_sev_conf->{'no-debug'};
> +	$policy |= 1 << 1 if $amd_sev_conf->{'no-key-sharing'};
> +	$policy |= 1 << 2 if $amd_sev_conf->{type} eq 'es';
> +	# disable migration with bit 3 nosend to prevent amd-sev-migration-attack
> +	$policy |= 1 << 3;
> +    } elsif ($amd_sev_conf->{type} eq 'snp') {
> +	$sev_mem_object .= 'sev-snp-guest,id=sev0';
> +	$sev_mem_object .= ',cbitpos='.$sev_hw_caps->{cbitpos};
> +	$sev_mem_object .= ',reduced-phys-bits='.$sev_hw_caps->{'reduced-phys-bits'};
> +
> +	# guest policy bit calculation as described in chapter 4.3:
> +	# https://www.amd.com/system/files/TechDocs/56860.pdf
> +	# Reserved bit must be one
> +	$policy = 1 << 17;
> +	$policy |= 1 << 16 if !defined($amd_sev_conf->{'allow-smt'}) || $amd_sev_conf->{'allow-smt'};
> +	$policy |= 1 << 19 if !$amd_sev_conf->{'no-debug'};
> +    }
> +
>   
>       $sev_mem_object .= ',policy='.sprintf("%#x", $policy);
>       $sev_mem_object .= ',kernel-hashes=on' if ($amd_sev_conf->{'kernel-hashes'});



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