Re: [PATCH v6 19/43] KVM: Let userspace disable per-VM mem attributes, enable per-gmem attributes

[Sean Christopherson]

On Thu, May 21, 2026, Fuad Tabba wrote:
> Hi Ackerley,
> 
> On Thu, 7 May 2026 at 21:22, Ackerley Tng via B4 Relay
> <devnull+ackerleytng.google.com@xxxxxxxxxx> wrote:
> >
> > From: Sean Christopherson <seanjc@xxxxxxxxxx>
> >
> > Make vm_memory_attributes a module parameter so that userspace can disable
> > the use of memory attributes on the VM level.
> >
> > To avoid inconsistencies in the way memory attributes are tracked in KVM
> > and guest_memfd, the vm_memory_attributes module_param is made
> > read-only (0444).
> >
> > Make CONFIG_KVM_VM_MEMORY_ATTRIBUTES selectable, only for (CoCo) VM types
> > that might use vm_memory_attributes.
> >
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> 
> Config files always confuse me, but Sashiko might be onto something:
> 
> https://sashiko.dev/#/patchset/20260507-gmem-inplace-conversion-v6-0-91ab5a8b19a4%40google.com?part=19

: Since this prompt does not have a default value, will it default to N
: and silently drop KVM_VM_MEMORY_ATTRIBUTES during configuration updates
: like make olddefconfig?
: 
: Existing userspace VMMs that rely on the KVM_SET_MEMORY_ATTRIBUTES ioctl
: for TDX or SEV VMs might fail to boot if the feature is unexpectedly
: compiled out. Could a default y be used to preserve backwards
: compatibility for existing configurations?

> I think this partially goes back to commit 6, the one I flagged
> yesterday. But also adding "default y" to KVM_VM_MEMORY_ATTRIBUTES?
> The default value should at least fix this issue, but I'm not sure if
> it would cause other problems...

Hrm.  As much as I want per-gmem attributes to be the default going forward,
silently breaking existing setups isn't great.  On the other hand, I'm *very*
skeptical there are any SNP or TDX deployments using a distro kernel, so I'm
still leaning towards forcing the issue and turning per-VM attributes off by
default.