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

[Philipp Giersfeld <philipp.giersfeld@canarybit.eu>]

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>
---

 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'});
-- 
2.39.5


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