[pve-devel] [PATCH qemu-server 1/1] QemuServer.pm: add support for storing VM memory in /dev/shm

[pve-devel] [PATCH qemu-server 1/1] QemuServer.pm: add support for storing VM memory in /dev/shm

[Drew Green via pve-devel]

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From: Drew Green <agreenbhm@gmail.com>
To: pve-devel@lists.proxmox.com
Subject: [PATCH qemu-server 1/1] QemuServer.pm: add support for storing VM memory in /dev/shm
Date: Wed, 19 Nov 2025 16:50:03 -0500
Message-ID: <CANDN8xF4U2-C=RGbJtn6y1i_ojHw0FrSc6y39+Rewha98rh8tg@mail.gmail.com>

PVE supports the VM option "ivshmem" which creates a memory-backed file on
the host operating system in /dev/shm to allow sharing resources between
the guest and host and other guests.  There is currently no support for
utilizing the Qemu feature to make the guest's system memory available via
the filesystem, which can be useful for experimentation and testing
purposes.  This patch adds an additional boolean option to the existing
"ivshmem" configuration called "ram", which when set to 1 will store the
guest's memory in the same location in /dev/shm that "ivshmem" normally
would.  Additional logic is added to support that option, such as adding an
additional "-machine" argument required for this and excluding the
"-device" argument normally required for traditional "ivshmem".

--- PVE/QemuServer.pm 2025-11-19 21:33:15.962832901 +0000
+++ PVE/QemuServer.pm 2025-11-19 21:36:40.390068876 +0000
@@ -199,6 +199,13 @@
         description =>
             "The name of the file. Will be prefixed with 'pve-shm-'.
Default is the VMID. Will be deleted when the VM is stopped.",
     },
+    ram => {
+ type => 'boolean',
+ optional => 1,
+ default => 0,
+ description =>
+    "Whether to use Inter-VM Shared Memory as backing for VM's RAM.",
+   },
 };

 my $audio_fmt = {
@@ -3711,17 +3718,21 @@
     if ($conf->{ivshmem}) {
         my $ivshmem = parse_property_string($ivshmem_fmt,
$conf->{ivshmem});

-        my $bus;
-        if ($q35) {
-            $bus = print_pcie_addr("ivshmem");
+ if (!$ivshmem->{ram}) {
+            my $bus;
+            if ($q35) {
+                $bus = print_pcie_addr("ivshmem");
+            } else {
+                $bus = print_pci_addr("ivshmem", $bridges, $arch);
+            }
+            push @$devices, '-device', "ivshmem-plain,memdev=ivshmem$bus,";
         } else {
-            $bus = print_pci_addr("ivshmem", $bridges, $arch);
-        }
+            push @$machineFlags, 'memory-backend=ivshmem';
+ }

         my $ivshmem_name = $ivshmem->{name} // $vmid;
         my $path = '/dev/shm/pve-shm-' . $ivshmem_name;

-        push @$devices, '-device', "ivshmem-plain,memdev=ivshmem$bus,";
         push @$devices, '-object',
             "memory-backend-file,id=ivshmem,share=on,mem-path=$path" .
",size=$ivshmem->{size}M";
     }

-- 
Drew Green
www.drewgreen.net
PGP: 17BDDD7E
<https://pgp.mit.edu/pks/lookup?op=get&search=0x37F838ED17BDDD7E>

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Re: [pve-devel] [PATCH qemu-server 1/1] QemuServer.pm: add support for storing VM memory in /dev/shm

[Fiona Ebner]

Hi,

Am 19.11.25 um 10:50 PM schrieb Drew Green via pve-devel:
> PVE supports the VM option "ivshmem" which creates a memory-backed file on
> the host operating system in /dev/shm to allow sharing resources between
> the guest and host and other guests.  There is currently no support for
> utilizing the Qemu feature to make the guest's system memory available via
> the filesystem, which can be useful for experimentation and testing
> purposes.  This patch adds an additional boolean option to the existing
> "ivshmem" configuration called "ram", which when set to 1 will store the
> guest's memory in the same location in /dev/shm that "ivshmem" normally
> would.  Additional logic is added to support that option, such as adding an
> additional "-machine" argument required for this and excluding the
> "-device" argument normally required for traditional "ivshmem".

yes, it is a rather niche use case. You can also add the machine flag
and object via custom '-args' in the VM configuration. Would using that
instead work for you? If yes, I'd rather postpone this, but feel free to
open a feature request on the bugzilla (referring back to your patch),
so we can better track interest from other users.

For completeness, regarding the patch submission: please use 'git
format-patch' to generate the email. And we'd also need a signed CLA and
a Signed-off-by trailer to be able to accept your patch. For details,
see: https://pve.proxmox.com/wiki/Developer_Documentation

Best Regards,
Fiona


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