RE: [PATCH v3 19/22] kvm: x86: Get/set expanded xstate buffer

[Wang, Wei W]

On Wednesday, December 29, 2021 8:39 AM, Sean Christopherson wrote:
> To: Liu, Jing2 <jing2.liu@xxxxxxxxx>
> Cc: x86@xxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> linux-doc@xxxxxxxxxxxxxxx; linux-kselftest@xxxxxxxxxxxxxxx; tglx@xxxxxxxxxxxxx;
> mingo@xxxxxxxxxx; bp@xxxxxxxxx; dave.hansen@xxxxxxxxxxxxxxx;
> pbonzini@xxxxxxxxxx; corbet@xxxxxxx; shuah@xxxxxxxxxx; Nakajima, Jun
> <jun.nakajima@xxxxxxxxx>; Tian, Kevin <kevin.tian@xxxxxxxxx>;
> jing2.liu@xxxxxxxxxxxxxxx; Zeng, Guang <guang.zeng@xxxxxxxxx>; Wang, Wei
> W <wei.w.wang@xxxxxxxxx>; Zhong, Yang <yang.zhong@xxxxxxxxx>
> Subject: Re: [PATCH v3 19/22] kvm: x86: Get/set expanded xstate buffer
> 
> Shortlog needs to have a verb somewhere.
> 
> On Wed, Dec 22, 2021, Jing Liu wrote:
> > From: Guang Zeng <guang.zeng@xxxxxxxxx>
> >
> > When AMX is enabled it requires a larger xstate buffer than the legacy
> > hardcoded 4KB one. Exising kvm ioctls
> 
> Existing
> 
> > (KVM_[G|S]ET_XSAVE under KVM_CAP_XSAVE) are not suitable for this
> > purpose.
> 
> ...
> 
> > Reuse KVM_SET_XSAVE for both old/new formats by reimplementing it to
> > do properly-sized memdup_user() based on the guest fpu container.
> 
> I'm confused, the first sentence says KVM_SET_XSAVE isn't suitable, the
> second says it can be reused with minimal effort.

Probably "doesn't support" sounds better than "isn't suitable" above. But plan to reword a bit:

With KVM_CAP_XSAVE, userspace uses a hardcoded 4KB buffer to get/set xstate data from/to
KVM. This doesn't work when dynamic features (e.g. AMX) are used by the guest, as KVM uses
a full expanded xstate buffer for the guest fpu emulation, which is larger than 4KB.

Add KVM_CAP_XSAVE2, and userspace gets the required xstate buffer size from KVM via
KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2). KVM_SET_XSAVE is extended with the support to
work with larger xstate data size passed from userspace. KVM_GET_XSAVE2 is preferred to
extending KVM_GET_XSAVE to work with large buffer size for backward-compatible considerations.
(Link: https://lkml.org/lkml/2021/12/15/510)

Also, update the api doc with the new KVM_GET_XSAVE2 ioctl.




> 
> > Also, update the api doc with the new KVM_GET_XSAVE2 ioctl.
> 
> ...
> 
> > @@ -5367,7 +5382,9 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
> >  		break;
> >  	}
> >  	case KVM_SET_XSAVE: {
> > -		u.xsave = memdup_user(argp, sizeof(*u.xsave));
> > +		int size = vcpu->arch.guest_fpu.uabi_size;
> 
> IIUC, reusing KVM_SET_XSAVE works by requiring that userspace use
> KVM_GET_XSAVE2 if userspace has expanded the guest FPU size by exposing
> relevant features to the guest via guest CPUID.  If so, then that needs to be
> enforced in KVM_GET_XSAVE, otherwise userspace will get subtle corruption
> by invoking the wrong ioctl, e.g.
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index
> 2c9606380bca..5d2acbd52df5 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5386,6 +5386,10 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
>                 break;
>         }
>         case KVM_GET_XSAVE: {
> +               r -EINVAL;
> +               if (vcpu->arch.guest_fpu.uabi_size > sizeof(struct
> kvm_xsave))
> +                       break;
> +

Looks good to me.

Thanks,
Wei