Re: [RFC PATCH 02/13] KVM: nSVM: Rework svm_flush_tlb_asid() to operate on a given VMCB

[Maxim Levitsky]

On Mon, 2025-03-03 at 21:58 +0000, Yosry Ahmed wrote:
> On Fri, Feb 28, 2025 at 08:29:34PM -0500, Maxim Levitsky wrote:
> > On Wed, 2025-02-05 at 18:23 +0000, Yosry Ahmed wrote:
> > > svm_flush_tlb_asid() currently operates on the current VMCB. In
> > > preparation for properly tracking TLB flushes for L1 and L2 ASIDs,
> > > refactor it to work on a given VMCB. All existing callers pass the
> > > current VMCB.
> > > 
> > > Create a svm_flush_tlb_guest() wrapper to use as the flush_tlb_guest()
> > > callback.
> > > 
> > > kvm_hv_vcpu_purge_flush_tlb() is only called when the current VMCB is
> > > passed to maintain current behavior.
> > > 
> > > No functional change intended.
> > > 
> > > Signed-off-by: Yosry Ahmed <yosry.ahmed@xxxxxxxxx>
> > > ---
> > >  arch/x86/kvm/svm/svm.c | 25 ++++++++++++++++++-------
> > >  1 file changed, 18 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > > index 08340ae57777b..2108b48ba4959 100644
> > > --- a/arch/x86/kvm/svm/svm.c
> > > +++ b/arch/x86/kvm/svm/svm.c
> > > @@ -3954,7 +3954,7 @@ static void svm_enable_nmi_window(struct kvm_vcpu *vcpu)
> > >  	svm->vmcb->save.rflags |= (X86_EFLAGS_TF | X86_EFLAGS_RF);
> > >  }
> > >  
> > > -static void svm_flush_tlb_asid(struct kvm_vcpu *vcpu)
> > > +static void svm_flush_tlb_asid(struct kvm_vcpu *vcpu, struct kvm_vmcb_info *vmcb)
> > >  {
> > >  	struct vcpu_svm *svm = to_svm(vcpu);
> > >  
> > > @@ -3963,7 +3963,8 @@ static void svm_flush_tlb_asid(struct kvm_vcpu *vcpu)
> > >  	 * A TLB flush for the current ASID flushes both "host" and "guest" TLB
> > >  	 * entries, and thus is a superset of Hyper-V's fine grained flushing.
> > >  	 */
> > > -	kvm_hv_vcpu_purge_flush_tlb(vcpu);
> > > +	if (vmcb == svm->current_vmcb)
> > > +		kvm_hv_vcpu_purge_flush_tlb(vcpu);
> > 
> > This is hyperv PV feature that should be looked upon very carefully.
> > 
> > To recap, 
> > each vCPU has 2 queues of pending TLB flush requests that target only small range of
> > memory pages. 
> 
> Thanks for pointing this out, I missed this.
> 
> > One is for L1 and one for L2, because now KVM supports a mode where L2
> > can ask L0 to do a tlb flush on its behalf, and KVM will figure out to which L1 vCPUs
> > to send this flush request.
> > 
> > Requests arrive from other vCPUs.
> > 
> > Here we purge the TLB request queue because we flushed a super-set of the requests,
> > which used to contain both L1 and L2 TLB, but soon that won't be true.
> > 
> > So I think it might make sense to also add vmcb to kvm_hv_vcpu_purge_flush_tlb, and then
> > depending if it is vmcb01 or vmcb02, purge the correct queue.
> > I don't know if this is theoretical or actual bug but it is better to be safe IMHO.
> 
> But I think we are already purging the right queue here. We purge the
> TLB flush requests only if we are flushing the current VMCB. Within
> kvm_hv_vcpu_purge_flush_tlb(), we choose the queue based on
> is_guest_mode(vcpu).
> 
> svm_flush_tlb_asid() is called when servicing a TLB flush request, at
> which point IIUC the current VMCB and whether the vCPU is in guest mode
> should be in sync. So we will be correctly purging the L1 or L2 queue
> based on the current VMCB.

Yes, I also think so, but to harden this code from a potential bug IMHO
it makes sense to ensure that svm_flush_tlb_asid works only on a given
vmcb without any hidden assumptions.

> 
> That being said, it's a bit confusing that svm_flush_tlb_asid() uses the
> VMCB to differentiate L1 and L2 ,while kvm_hv_vcpu_purge_flush_tlb()
> uses is_guest_mode(). We also miss the opportunity to purge both queues
> when called from svm_flush_tlb_all().

Yes, I noticed that too.

> 
> However, we cannot pass the VMCB to kvm_hv_vcpu_purge_flush_tlb() as it
> is also called from common code. So I think we can make
> kvm_hv_vcpu_purge_flush_tlb() take is_guest_mode as a parameter and pass
> it here based on which VMCB is passed in.



> 
> WDYT?
> 


Looking at this again, I see that kvm_hv_vcpu_purge_flush_tlb() can't really work
on a vmcb, so maybe the better solution is to remove the call to 
kvm_hv_vcpu_purge_flush_tlb() from svm_flush_tlb_asid_vmcb at all
and instead let the caller call both svm_flush_tlb_asid() and kvm_hv_vcpu_purge_flush_tlb()?

This might lead to some code duplication but this will put emphasis that svm_flush_tlb_asid_vmcb
can work on any vmcb regardless of which one is loaded and such.

As long as it works though, I won't strongly object to whatever code that works.

Best regards,
	Maxim Levitsky