[PATCH RFC] kvm: x86: AVX512_BF16 feature support

[PATCH RFC] kvm: x86: AVX512_BF16 feature support

[Jing Liu]

The patch focuses on a new instruction AVX512_BF16 support for kvm guest, defined
as CPUID.(EAX=7,ECX=1):EAX[bit 5], see spec[1].

The kvm implementation depends on kernel patch[2] which is in lkml discussion.

References:
[1] https://software.intel.com/sites/default/files/managed/c5/15/\
    architecture-instruction-set-extensions-programming-reference.pdf
[2] https://lkml.org/lkml/2019/6/19/912

Jing Liu (1):
  kvm: x86: Expose AVX512_BF16 feature to guest

 arch/x86/kvm/cpuid.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

-- 
1.8.3.1

[PATCH RFC] kvm: x86: Expose AVX512_BF16 feature to guest

[Jing Liu]

AVX512 BFLOAT16 instructions support 16-bit BFLOAT16 floating-point
format (BF16) for deep learning optimization.

Intel adds AVX512 BFLOAT16 feature in CooperLake, which is CPUID.7.1.EAX[5].

Detailed information of the CPUID bit can be found here,
https://software.intel.com/sites/default/files/managed/c5/15/\
architecture-instruction-set-extensions-programming-reference.pdf.

Signed-off-by: Jing Liu <jing2.liu@linux.intel.com>
---
 arch/x86/kvm/cpuid.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index e18a9f9..10be53f 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -484,6 +484,7 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
 		entry->edx = 0;
 		break;
 	case 7: {
+		int i, times = entry->eax;
 		entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
 		/* Mask ebx against host capability word 9 */
 		if (index == 0) {
@@ -507,12 +508,23 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
 			 * if the host doesn't support it.
 			 */
 			entry->edx |= F(ARCH_CAPABILITIES);
-		} else {
+		} else if (index > times) {
+			entry->eax = 0;
 			entry->ebx = 0;
 			entry->ecx = 0;
 			entry->edx = 0;
 		}
-		entry->eax = 0;
+		for (i = 1; i <= times; i++) {
+			if (*nent >= maxnent)
+				goto out;
+			do_cpuid_1_ent(&entry[i], function, i);
+			entry[i].eax &= F(AVX512_BF16);
+			entry[i].ebx = 0;
+			entry[i].ecx = 0;
+			entry[i].edx = 0;
+			entry[i].flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
+			++*nent;
+		}
 		break;
 	}
 	case 9:
-- 
1.8.3.1

Re: [PATCH RFC] kvm: x86: Expose AVX512_BF16 feature to guest

[Paolo Bonzini]

On 20/06/19 13:21, Jing Liu wrote:
> +		for (i = 1; i <= times; i++) {
> +			if (*nent >= maxnent)
> +				goto out;
> +			do_cpuid_1_ent(&entry[i], function, i);
> +			entry[i].eax &= F(AVX512_BF16);
> +			entry[i].ebx = 0;
> +			entry[i].ecx = 0;
> +			entry[i].edx = 0;
> +			entry[i].flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
> +			++*nent;

This woud be wrong for i > 1, so instead make this

	if (entry->eax >= 1)

and define F(AVX512_BF16) as a new constant kvm_cpuid_7_1_eax_features.

Paolo

Re: [PATCH RFC] kvm: x86: Expose AVX512_BF16 feature to guest

[Liu, Jing2]

Hi Paolo,

On 6/20/2019 8:16 PM, Paolo Bonzini wrote:
> On 20/06/19 13:21, Jing Liu wrote:
>> +		for (i = 1; i <= times; i++) {
>> +			if (*nent >= maxnent)
>> +				goto out;
>> +			do_cpuid_1_ent(&entry[i], function, i);
>> +			entry[i].eax &= F(AVX512_BF16);
>> +			entry[i].ebx = 0;
>> +			entry[i].ecx = 0;
>> +			entry[i].edx = 0;
>> +			entry[i].flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
>> +			++*nent;
> 
> This woud be wrong for i > 1, so instead make this
> 
> 	if (entry->eax >= 1)
> 

I am confused about the @index parameter. @index seems not used for
every case except 0x07. Since the caller function only has @index=0, so
all other cases except 0x07 put cpuid info from subleaf=0 to max subleaf.

What do you think about @index in current function? Does it mean, we
need put cpuid from index to max subleaf to @entry[i]? If so, the logic
seems as follows,

if (index == 0) {
     // Put subleaf 0 into @entry
     // Put subleaf 1 into @entry[1]
} else if (index < entry->eax) {
     // Put subleaf 1 into @entry
} else {
     // Put all zero into @entry
}

But this seems not identical with other cases, for current caller
function. Or we can simply ignore @index in 0x07 and just put all possible
subleaf info back?

> and define F(AVX512_BF16) as a new constant kvm_cpuid_7_1_eax_features.
> 
Got it.


Thanks,
Jing

> Paolo
> 

Re: [PATCH RFC] kvm: x86: Expose AVX512_BF16 feature to guest

[Jing Liu]

Hi Paolo,

After thinking more, I found way to satisfy all cases in a easy way.
How about things like this?

@@ -507,12 +510,26 @@ static inline int __do_cpuid_ent(struct 
kvm_cpuid_entry2 *entry, u32 fu
                          * if the host doesn't support it.
                          */
                         entry->edx |= F(ARCH_CAPABILITIES);
+               } else if (index == 1) {
+                       entry->eax &= kvm_cpuid_7_1_eax_x86_features;
+                       entry->ebx = 0;
+                       entry->ecx = 0;
+                       entry->edx = 0;
                 } else {
+                       entry->eax = 0;
                         entry->ebx = 0;
                         entry->ecx = 0;
                         entry->edx = 0;
                 }
-               entry->eax = 0;
+
+               if (index == 0 && entry->eax >= 1) {
+                       entry[1].eax &= kvm_cpuid_7_1_eax_x86_features;
+                       entry[1].ebx = 0;
+                       entry[1].ecx = 0;
+                       entry[1].edx = 0;
+                       entry[1].flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
+                       ++*nent;
+               }
                 break;
         }


Or you prefer that I update this into another version later?

Thanks!
Jing

On 6/20/2019 11:09 PM, Liu, Jing2 wrote:
> Hi Paolo,
> 
> On 6/20/2019 8:16 PM, Paolo Bonzini wrote:
>> On 20/06/19 13:21, Jing Liu wrote:
>>> +        for (i = 1; i <= times; i++) {
>>> +            if (*nent >= maxnent)
>>> +                goto out;
>>> +            do_cpuid_1_ent(&entry[i], function, i);
>>> +            entry[i].eax &= F(AVX512_BF16);
>>> +            entry[i].ebx = 0;
>>> +            entry[i].ecx = 0;
>>> +            entry[i].edx = 0;
>>> +            entry[i].flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
>>> +            ++*nent;
>>
>> This woud be wrong for i > 1, so instead make this
>>
>>     if (entry->eax >= 1)
>>
> 
> I am confused about the @index parameter. @index seems not used for
> every case except 0x07. Since the caller function only has @index=0, so
> all other cases except 0x07 put cpuid info from subleaf=0 to max subleaf.
> 
> What do you think about @index in current function? Does it mean, we
> need put cpuid from index to max subleaf to @entry[i]? If so, the logic
> seems as follows,
> 
> if (index == 0) {
>      // Put subleaf 0 into @entry
>      // Put subleaf 1 into @entry[1]
> } else if (index < entry->eax) {
>      // Put subleaf 1 into @entry
> } else {
>      // Put all zero into @entry
> }
> 
> But this seems not identical with other cases, for current caller
> function. Or we can simply ignore @index in 0x07 and just put all possible
> subleaf info back?
> 
>> and define F(AVX512_BF16) as a new constant kvm_cpuid_7_1_eax_features.
>>
> Got it.
> 
> 
> Thanks,
> Jing
> 
>> Paolo
>>

Re: [PATCH RFC] kvm: x86: Expose AVX512_BF16 feature to guest

[Paolo Bonzini]

On 24/06/19 05:10, Jing Liu wrote:
>> What do you think about @index in current function? Does it mean, we
>> need put cpuid from index to max subleaf to @entry[i]? If so, the logic
>> seems as follows,
>>
>> if (index == 0) {
>>      // Put subleaf 0 into @entry
>>      // Put subleaf 1 into @entry[1]
>> } else if (index < entry->eax) {
>>      // Put subleaf 1 into @entry
>> } else {
>>      // Put all zero into @entry
>> }
>>
>> But this seems not identical with other cases, for current caller
>> function. Or we can simply ignore @index in 0x07 and just put all
>> possible subleaf info back?

There are indeed quite some cleanups to be made there.  Let me post a
series as soon as possible, and you can base your work on it.

Paolo

Re: [PATCH RFC] kvm: x86: Expose AVX512_BF16 feature to guest

[Jing Liu]

Hi Paolo,

On 6/24/2019 4:33 PM, Paolo Bonzini wrote:
> On 24/06/19 05:10, Jing Liu wrote:
>>> What do you think about @index in current function? Does it mean, we
>>> need put cpuid from index to max subleaf to @entry[i]? If so, the logic
>>> seems as follows,
>>>
>>> if (index == 0) {
>>>       // Put subleaf 0 into @entry
>>>       // Put subleaf 1 into @entry[1]
>>> } else if (index < entry->eax) {
>>>       // Put subleaf 1 into @entry
>>> } else {
>>>       // Put all zero into @entry
>>> }
>>>
>>> But this seems not identical with other cases, for current caller
>>> function. Or we can simply ignore @index in 0x07 and just put all
>>> possible subleaf info back?
> 
> There are indeed quite some cleanups to be made there.  Let me post a
> series as soon as possible, and you can base your work on it.
> 

Thanks. I just had another mail (replying you in this serial) appending
some codes to deal with case 7. If you prefer to firstly cleanup, I can
wait for the patch then. :)

Thanks,
Jing

> Paolo
>