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Subject: Re: [PATCH RFC 4/7] kvm: x86: Add new ioctls for XSAVE extension
From:   "Liu, Jing2" <jing2.liu@linux.intel.com>
To:     Sean Christopherson <seanjc@google.com>
Cc:     pbonzini@redhat.com, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, jing2.liu@intel.com
References: <20210207154256.52850-1-jing2.liu@linux.intel.com>
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 <920df897-56d8-1f81-7ce2-0050fb744bd7@linux.intel.com>
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Message-ID: <7cbcbcf6-770d-8ac3-e7f3-6fed3331aa00@linux.intel.com>
Date:   Mon, 7 Jun 2021 13:23:27 +0800
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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@linux.intel.com>
>>>>> ---
>>>>>    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