Re: [PATCH v2 4/8] KVM: x86: Add X86EMUL_F_INVTLB and pass it in em_invlpg()

[Binbin Wu <binbin.wu@linux.intel.com>]

On 8/16/2023 7:11 AM, Sean Christopherson wrote:
> On Wed, Jul 19, 2023, Zeng Guang wrote:
>> From: Binbin Wu <binbin.wu@linux.intel.com>
>>
>> Add an emulation flag X86EMUL_F_INVTLB, which is used to identify an
>> instruction that does TLB invalidation without true memory access.
>>
>> Only invlpg & invlpga implemented in emulator belong to this kind.
>> invlpga doesn't need additional information for emulation. Just pass
>> the flag to em_invlpg().
> Please add a paragraph explaining *why* this flag is being added.  Ideally, the
> previous patch would also explain the need for an IMPLICIT flag, but that one
> doesn't bug me all that much because implicit accesses are known to be special
> snowflakes, i.e. it's easy to imagine that KVM would need to identify such
> accesses.  But for INVLPG, without already knowing the details of LASS (or LAM),
> it's harder to think of why it needs to exist.
OK, will add the reason for this case and for IMPLICIT as well.
Thanks.


>
>> Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
>> Signed-off-by: Zeng Guang <guang.zeng@intel.com>
>> ---
>>   arch/x86/kvm/emulate.c     | 4 +++-
>>   arch/x86/kvm/kvm_emulate.h | 1 +
>>   2 files changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
>> index 8e706d19ae45..9b4b3ce6d52a 100644
>> --- a/arch/x86/kvm/emulate.c
>> +++ b/arch/x86/kvm/emulate.c
>> @@ -3443,8 +3443,10 @@ static int em_invlpg(struct x86_emulate_ctxt *ctxt)
>>   {
>>   	int rc;
>>   	ulong linear;
>> +	unsigned max_size;
> 	unsigned int
Let me think why I use 'unsigned'...
It's because the exist code uses 'unsigned'.
I suppose it is considered bad practice?
I will cleanup the exist code as well. Is it OK to cleanup it 
opportunistically inside this patch?


>> -	rc = linearize(ctxt, ctxt->src.addr.mem, 1, false, &linear);
>> +	rc = __linearize(ctxt, ctxt->src.addr.mem, &max_size, 1, ctxt->mode,
>> +		&linear, X86EMUL_F_INVTLB);
> Align indentation:
Will update it.

>
> 	rc = __linearize(ctxt, ctxt->src.addr.mem, &max_size, 1, ctxt->mode,
> 			 &linear, X86EMUL_F_INVTLB);
>
>>   	if (rc == X86EMUL_CONTINUE)
>>   		ctxt->ops->invlpg(ctxt, linear);
>>   	/* Disable writeback. */
>> diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
>> index c0e48f4fa7c4..c944055091e1 100644
>> --- a/arch/x86/kvm/kvm_emulate.h
>> +++ b/arch/x86/kvm/kvm_emulate.h
>> @@ -93,6 +93,7 @@ struct x86_instruction_info {
>>   #define X86EMUL_F_FETCH			BIT(1)
>>   #define X86EMUL_F_BRANCH		BIT(2)
>>   #define X86EMUL_F_IMPLICIT		BIT(3)
>> +#define X86EMUL_F_INVTLB		BIT(4)
> Why F_INVTLB instead of X86EMUL_F_INVLPG?  Ah, because LAM is ignored for the
> linear address in the INVPCID and INVVPID descriptors.  Hrm.
>
> I think my vote is to call this X86EMUL_F_INVLPG even though *in theory* it's not
> strictly limited to INVLPG.  Odds are good KVM's emulator will never support
> INVPCID or INVVPID,
One case is kvm_handle_invpcid() is in the common kvm x86 code.
LAM doesn't apply to the address in descriptor of invpcid though, but I 
am not sure if there will be the need for SVM in the future.
But for now, F_INVLPG is OK if you think F_INVTLB brings confusion.


> and IMO even though F_INVLPG would be somewhat of a misnomer,
> it's much more intuitive even for INVPCID and INVVPID descriptors.  F_INVTLB makes
> me think more of the actual act of invalidating the TLB.
>
> I'm not dead set against INVTLB if someone really likes it, but I did scratch my
> head for a second when I saw it.