Re: [PATCH RFC v4 10/44] KVM: guest_memfd: Add support for KVM_SET_MEMORY_ATTRIBUTES2

[Michael Roth]

On Tue, Apr 14, 2026 at 06:37:00PM -0500, Michael Roth wrote:
> On Wed, Apr 01, 2026 at 03:38:12PM -0700, Ackerley Tng wrote:
> > Michael Roth <michael.roth@xxxxxxx> writes:
> > 
> > >
> > > [...snip...]
> > >
> > >>  static unsigned long kvm_get_vm_memory_attributes(struct kvm *kvm, gfn_t gfn)
> > >>  {
> > >> @@ -2635,6 +2625,8 @@ static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
> > >>  		return -EINVAL;
> > >>  	if (!PAGE_ALIGNED(attrs->address) || !PAGE_ALIGNED(attrs->size))
> > >>  		return -EINVAL;
> > >> +	if (attrs->error_offset)
> > >> +		return -EINVAL;
> > >>  	for (i = 0; i < ARRAY_SIZE(attrs->reserved); i++) {
> > >>  		if (attrs->reserved[i])
> > >>  			return -EINVAL;
> > >> @@ -4983,6 +4975,11 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
> > >>  		return 1;
> > >>  	case KVM_CAP_GUEST_MEMFD_FLAGS:
> > >>  		return kvm_gmem_get_supported_flags(kvm);
> > >> +	case KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES:
> > >> +		if (vm_memory_attributes)
> > >> +			return 0;
> > >> +
> > >> +		return kvm_supported_mem_attributes(kvm);
> > >
> > > Based on the discussion from the PUCK call this morning,
> > 
> > Thanks for copying the discussion here, I'll start attending PUCK to
> > catch those discussions too :)
> > 
> > > it sounds like it
> > > would be a good idea to limit kvm_supported_mem_attributes() to only
> > > reporting KVM_MEMORY_ATTRIBUTE_PRIVATE if the underlying CoCo
> > > implementation has all the necessary enablement to support in-place
> > > conversion via guest_memfd. In the case of SNP, there is a
> > > documentation/parameter check in snp_launch_update() that needs to be
> > > relaxed in order for userspace to be able to pass in a NULL 'src'
> > > parameter (since, for in-place conversion, it would be initialized in place
> > > as shared memory prior to the call, since by the time kvm_gmem_poulate()
> > > it will have been set to private and therefore cannot be faulted in via
> > > GUP (and if it could, we'd be unecessarily copying the src back on top
> > > of itself since src/dst are the same).
> > 
> > Could this be a separate thing? If I'm understanding you correctly, it's
> > not strictly a requirement for snp_launch_update() to first support a
> > NULL 'src' parameter before this series lands.
> 
> I think we are already sync'd up on this during PUCK, but for the benefit
> of others: Sean pointed out that if we don't then we'll need to add yet
> another capability so userspace can determine when it can actually do
> in-place conversion for SNP.

(in-place conversion for SNP during pre-launch/populate phase, I meant)

> 
> Right now, this series effectively advertises in place conversion at the
> point where KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES reports
> 'KVM_MEMORY_ATTRIBUTE_PRIVATE', so I slightly reworked the series to
> include the snp_launch_update() change prior to that point in time in
> the series. Thanks to prereqs and changes/requirements you've already
> pulled in, it's just one additional patch now:
> 
>  KVM: SEV: Make 'uaddr' parameter optional for KVM_SEV_SNP_LAUNCH_UPDATE 
> 
> I also did some minor updates (prefixed with a "[squash]" tag) to advertise
> the KVM_SET_MEMORY_ATTRIBUTES2_PRESERVED flag so it can be used by

Though I'm not sure how we deal with it if SNP/TDX at some point become
capable of using the PRESERVED flag *after* populate... but maybe that's
too unlikely to worry about? If we wanted to address it though, we could
have both PRESERVED and PRESERVED_BEFORE_LAUNCH so they can be
enumerated separately from the start.

> userspace for SNP/TDX in the kvm_gmem_populate() path as agreed upon
> during PUCK.
> 
> The branch is here, with the patches moved to where I think they
> should remain (or be squashed in for the [squash] ones):
> 
>   https://github.com/AMDESE/linux/commits/guest_memfd-inplace-conversion-v4-snp2/
> 
> I've also updated the QEMU patches to use the agreed-upon API flow and
> pushed them here:
> 
>   https://github.com/AMDESE/qemu/commits/snp-inplace-for-v4-wip2/
> 
> To start an SNP guest with in-place conversion:
> 
>   qemu-system-x86 \
>   -machine q35,confidential-guest-support=sev0,memory-backend=ram1 \
>   -object sev-snp-guest,id=sev0,...,convert-in-place=true \
>   -object memory-backend-memfd,id=ram1,size=16G,share=true,reserve=false

Sorry, that should've been:

  -object memory-backend-guest-memfd,id=ram1,size=16G,share=true,reserve=false

> 
> To start an normal non-CoCo guest backed by guest_memfd with shared memory:
> 
>   qemu-system-x86 \
>   -machine q35,confidential-guest-support=sev0,memory-backend=ram1 \
>   -object memory-backend-memfd,id=ram1,size=16G,share=true,reserve=false

and:

  -object memory-backend-guest-memfd,id=ram1,size=16G,share=true,reserve=false

(and both require kvm.vm_memory_attributes=0)

-Mike

> 
> Thanks,
> 
> Mike