Re: [PATCH 4/4] KVM: hyper-v: Advertise support for fast XMM hypercalls

[Vitaly Kuznetsov]

Siddharth Chandrasekaran <sidcha@xxxxxxxxx> writes:

> On Thu, Apr 08, 2021 at 02:05:53PM +0200, Vitaly Kuznetsov wrote:
>> Siddharth Chandrasekaran <sidcha@xxxxxxxxx> writes:
>>
>> > Now that all extant hypercalls that can use XMM registers (based on
>> > spec) for input/outputs are patched to support them, we can start
>> > advertising this feature to guests.
>> >
>> > Cc: Alexander Graf <graf@xxxxxxxxxx>
>> > Cc: Evgeny Iakovlev <eyakovl@xxxxxxxxx>
>> > Signed-off-by: Siddharth Chandrasekaran <sidcha@xxxxxxxxx>
>> > ---
>> >  arch/x86/include/asm/hyperv-tlfs.h | 4 ++--
>> >  arch/x86/kvm/hyperv.c              | 1 +
>> >  2 files changed, 3 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/arch/x86/include/asm/hyperv-tlfs.h b/arch/x86/include/asm/hyperv-tlfs.h
>> > index e6cd3fee562b..1f160ef60509 100644
>> > --- a/arch/x86/include/asm/hyperv-tlfs.h
>> > +++ b/arch/x86/include/asm/hyperv-tlfs.h
>> > @@ -49,10 +49,10 @@
>> >  /* Support for physical CPU dynamic partitioning events is available*/
>> >  #define HV_X64_CPU_DYNAMIC_PARTITIONING_AVAILABLE    BIT(3)
>> >  /*
>> > - * Support for passing hypercall input parameter block via XMM
>> > + * Support for passing hypercall input and output parameter block via XMM
>> >   * registers is available
>> >   */
>> > -#define HV_X64_HYPERCALL_PARAMS_XMM_AVAILABLE                BIT(4)
>> > +#define HV_X64_HYPERCALL_PARAMS_XMM_AVAILABLE                BIT(4) | BIT(15)
>>
>> TLFS 6.0b states that there are two distinct bits for input and output:
>>
>> CPUID Leaf 0x40000003.EDX:
>> Bit 4: support for passing hypercall input via XMM registers is available.
>> Bit 15: support for returning hypercall output via XMM registers is available.
>>
>> and HV_X64_HYPERCALL_PARAMS_XMM_AVAILABLE is not currently used
>> anywhere, I'd suggest we just rename
>>
>> HV_X64_HYPERCALL_PARAMS_XMM_AVAILABLE to HV_X64_HYPERCALL_XMM_INPUT_AVAILABLE
>> and add HV_X64_HYPERCALL_XMM_OUTPUT_AVAILABLE (bit 15).
>
> That is how I had it initially; but then noticed that we would never
> need to use either of them separately. So it seemed like a reasonable
> abstraction to put them together.
>

Actually, we may. In theory, KVM userspace may decide to expose just
one of these two to the guest as it is not obliged to copy everything
from KVM_GET_SUPPORTED_HV_CPUID so we will need separate
guest_cpuid_has() checks.

(This reminds me of something I didn't see in your series:
we need to check that XMM hypercall parameters support was actually
exposed to the guest as it is illegal for a guest to use it otherwise --
and we will likely need two checks, for input and output).

Also, (and that's what triggered my comment) all other HV_ACCESS_* in
kvm_get_hv_cpuid() are single bits so my first impression was that you
forgot one bit, but then I saw that you combined them together.

-- 
Vitaly