Re: [RFC PATCH 0/7] Support protection keys in an AMD EPYC-Milan VM

[David Edmondson <dme@dme.org>]

On Friday, 2021-06-11 at 18:01:55 +02, Paolo Bonzini wrote:

> First of all, sorry for the delayed review.
>
> On 20/05/21 16:56, David Edmondson wrote:
>> AMD EPYC-Milan CPUs introduced support for protection keys, previously
>> available only with Intel CPUs.
>> 
>> AMD chose to place the XSAVE state component for the protection keys
>> at a different offset in the XSAVE state area than that chosen by
>> Intel.
>> 
>> To accommodate this, modify QEMU to behave appropriately on AMD
>> systems, allowing a VM to properly take advantage of the new feature.
>
> Uff, that sucks. :(
>
> If I understand correctly, the problem is that the layout of 
> KVM_GET_XSAVE/KVM_SET_XSAVE depends on the host CPUID, which in 
> retrospect would be obvious.  Is that correct?

Yes.

> If so, it would make sense and might even be easier to drop all usage
> of X86XSaveArea:
>
> * update ext_save_areas based on CPUID information in kvm_cpu_instance_init
>
> * make x86_cpu_xsave_all_areas and x86_cpu_xrstor_all_areas use the 
> ext_save_areas offsets to build pointers to XSaveAVX, XSaveBNDREG, etc.
>
> What do you think?

I will produce a patch and send it out.

> Paolo
>
>> Further, avoid manipulating XSAVE state components that are not
>> present on AMD systems.
>> 
>> The code in patch 6 that changes the CPUID 0x0d leaf is mostly dumped
>> somewhere that seemed to work - I'm not sure where it really belongs.
>> 
>> David Edmondson (7):
>>    target/i386: Declare constants for XSAVE offsets
>>    target/i386: Use constants for XSAVE offsets
>>    target/i386: Clarify the padding requirements of X86XSaveArea
>>    target/i386: Prepare for per-vendor X86XSaveArea layout
>>    target/i386: Introduce AMD X86XSaveArea sub-union
>>    target/i386: Adjust AMD XSAVE PKRU area offset in CPUID leaf 0xd
>>    target/i386: Manipulate only AMD XSAVE state on AMD
>> 
>>   target/i386/cpu.c            | 19 +++++----
>>   target/i386/cpu.h            | 80 ++++++++++++++++++++++++++++--------
>>   target/i386/kvm/kvm.c        | 57 +++++++++----------------
>>   target/i386/tcg/fpu_helper.c | 20 ++++++---
>>   target/i386/xsave_helper.c   | 70 +++++++++++++++++++------------
>>   5 files changed, 152 insertions(+), 94 deletions(-)
>> 

dme.
-- 
Oliver darling, call Mister Haney, I think our speakers are blown.