Re: [PATCH RFC 4/7] kvm: x86: Add new ioctls for XSAVE extension

[Liu, Jing2]

On 6/1/2021 6:24 PM, Liu, Jing2 wrote:
> On 5/26/2021 10:43 PM, Sean Christopherson wrote:
> > On Wed, May 26, 2021, Liu, Jing2 wrote:
> > > On 5/25/2021 5:50 AM, Sean Christopherson wrote:
> > > > On Sun, Feb 07, 2021, Jing Liu wrote:
> > > > > The static xstate buffer kvm_xsave contains the extended register
> > > > > states, but it is not enough for dynamic features with large state.
> > > > >
> > > > > Introduce a new capability called KVM_CAP_X86_XSAVE_EXTENSION to
> > > > > detect if hardware has XSAVE extension (XFD). Meanwhile, add two
> > > > > new ioctl interfaces to get/set the whole xstate using struct
> > > > > kvm_xsave_extension buffer containing both static and dynamic
> > > > > xfeatures. Reuse fill_xsave and load_xsave for both cases.
> > > > >
> > > > > Signed-off-by: Jing Liu <jing2.liu@xxxxxxxxxxxxxxx>
> > > > > ---
> > > > >    arch/x86/include/uapi/asm/kvm.h |  5 +++
> > > > >    arch/x86/kvm/x86.c              | 70 
> > > > > +++++++++++++++++++++++++--------
> > > > >    include/uapi/linux/kvm.h        |  8 ++++
> > > > >    3 files changed, 66 insertions(+), 17 deletions(-)
> > > > >
> > > > > diff --git a/arch/x86/include/uapi/asm/kvm.h 
> > > > > b/arch/x86/include/uapi/asm/kvm.h
> > > > > index 89e5f3d1bba8..bf785e89a728 100644
> > > > > --- a/arch/x86/include/uapi/asm/kvm.h
> > > > > +++ b/arch/x86/include/uapi/asm/kvm.h
> > > > > @@ -362,6 +362,11 @@ struct kvm_xsave {
> > > > >        __u32 region[1024];
> > Hold up a sec.  How big is the AMX data?
> AMX tileconfig size is 64B, but tiledata size is 8K.
> > The existing size is 4096 bytes, not
> > 1024 bytes.  IIRC, AMX is >4k, so we still need a new ioctl(),
> Yep, kvm_xsave can hold 4KB state. We need a new ioctl, holding all 
> the states,
> not only AMX. And once KVM supports AMX, the size will >4096 so qemu need
> use kvm_xsave2 instead, otherwise, cannot save/restore whole AMX state.
> > but we should be
> > careful to mentally adjust for the __u32 when mentioning the sizes.
> >
> > > > >    };
> > > > > +/* for KVM_CAP_XSAVE_EXTENSION */
> > > > > +struct kvm_xsave_extension {
> > > > > +    __u32 region[3072];
> > > > Fool me once, shame on you (Intel).  Fool me twice, shame on me (KVM).
> > > >
> > > > As amusing as kvm_xsave_really_extended would be, the required size 
> > > > should be
> > > > discoverable, not hardcoded.
> > > Thanks for reviewing the patch.  When looking at current kvm_xsave 
> > > structure,
> > > I felt confusing about the static hardcoding of 1024 bytes, but 
> > > failed to
> > > find clue for its final decision in 2010[1].
> > Simplicitly and lack of foresight :-)
> >
> > > So we'd prefer to changing the way right? Please correct me if I 
> > > misunderstood.
> > Sadly, we can't fix the existing ioctl() without breaking userspace.  
> > But for
> > the new ioctl(), yes, its size should not be hardcoded.
> >
> > > > Nothing prevents a hardware vendor from inventing a newfangled 
> > > > feature that
> > > > requires yet more space.  As an alternative to adding a dedicated
> > > > capability, can we leverage GET_SUPPORTED_CPUID, leaf CPUID.0xD,
> > > Yes, this is a good way to avoid a dedicated capability. Thanks for the
> > > suggestion.  Use 0xD.1.EBX for size of enabled xcr0|xss if supposing
> > > kvm_xsave cares both.
I think kvm_xsave ioctl only cares user states because supervisor states 
should
always use compacted format. When trying to think about how to get/set 
supervisor
states, I think it can not reuse current design (qemu talks with kvm via 
a buffer
and set/get to/from qemu's env->some_feature one by one according to the 
offset).
So do we need handle supervisor states and change the way?

For kvm_xsave2 which expands to >4096B, if reuse and expand current way, 
it only
detects xcr0 from 0xD.0.EBX.

[...]

Thanks,
Jing