Re: [RFC PATCH 13/24] KVM: nSVM: Parameterize svm_flush_tlb_asid() by is_guest_mode

[Yosry Ahmed <yosry.ahmed@linux.dev>]

On Thu, Apr 03, 2025 at 04:10:06PM -0400, Maxim Levitsky wrote:
> On Wed, 2025-03-26 at 19:36 +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 take is_guest_mode and find the proper VMCB. All existing
> > callers pass is_guest_mode(vcpu) to maintain existing behavior for now.
> > 
> > Move the comment about only flushing the current ASID to
> > svm_flush_tlb_all(), where it probably should have been anyway, because
> > svm_flush_tlb_asid() now flushes a given ASID, not the current ASID.
> > 
> > Create a svm_flush_tlb_guest() wrapper to use as the flush_tlb_guest()
> > callback.
> > 
> > No functional change intended.
> > 
> > Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
> > ---
> >  arch/x86/kvm/svm/svm.c | 39 +++++++++++++++++++++++++--------------
> >  1 file changed, 25 insertions(+), 14 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index 865c5ce4fa473..fb6b9f88a1504 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -4016,25 +4016,24 @@ 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 struct vmcb *svm_get_vmcb(struct vcpu_svm *svm, bool is_guest_mode)
> > +{
> > +	return is_guest_mode ? svm->nested.vmcb02.ptr : svm->vmcb01.ptr;
> > +}
> 
> Not sure 100% about this helper, it name might be a bit confusing because
> we already have a current vmcb. Maybe add a comment above stating this
> this is to get vmcb which might not be currently active?

Yeah I spent some time trying to come up with an elaborate name then
convinced myself that the is_guest_mode parameter will make it clear
that the caller specifies which VMCB it wants, regardless of which VMCB
is current.

I can add a comment to make it clearer.

> 
> > +
> > +static void svm_flush_tlb_asid(struct kvm_vcpu *vcpu, bool is_guest_mode)
> >  {
> >  	struct vcpu_svm *svm = to_svm(vcpu);
> > +	struct vmcb *vmcb = svm_get_vmcb(svm, is_guest_mode);
> >  
> >  	/*
> >  	 * Unlike VMX, SVM doesn't provide a way to flush only NPT TLB entries.
> >  	 * 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, is_guest_mode(vcpu));
> > -
> > -	/*
> > -	 * Flush only the current ASID even if the TLB flush was invoked via
> > -	 * kvm_flush_remote_tlbs().  Although flushing remote TLBs requires all
> > -	 * ASIDs to be flushed, KVM uses a single ASID for L1 and L2, and
> > -	 * unconditionally does a TLB flush on both nested VM-Enter and nested
> > -	 * VM-Exit (via kvm_mmu_reset_context()).
> > -	 */
> > -	vmcb_set_flush_asid(svm->vmcb);
> > +	kvm_hv_vcpu_purge_flush_tlb(vcpu, is_guest_mode);
> > +	if (vmcb)
> > +		vmcb_set_flush_asid(vmcb);
> >  }
> >  
> >  static void svm_flush_tlb_current(struct kvm_vcpu *vcpu)
> > @@ -4050,7 +4049,7 @@ static void svm_flush_tlb_current(struct kvm_vcpu *vcpu)
> >  	if (svm_hv_is_enlightened_tlb_enabled(vcpu) && VALID_PAGE(root_tdp))
> >  		hyperv_flush_guest_mapping(root_tdp);
> >  
> > -	svm_flush_tlb_asid(vcpu);
> > +	svm_flush_tlb_asid(vcpu, is_guest_mode(vcpu));
> >  }
> >  
> >  static void svm_flush_tlb_all(struct kvm_vcpu *vcpu)
> > @@ -4065,7 +4064,14 @@ static void svm_flush_tlb_all(struct kvm_vcpu *vcpu)
> >  	if (WARN_ON_ONCE(svm_hv_is_enlightened_tlb_enabled(vcpu)))
> >  		hv_flush_remote_tlbs(vcpu->kvm);
> >  
> > -	svm_flush_tlb_asid(vcpu);
> > +	/*
> > +	 * Flush only the current ASID even if the TLB flush was invoked via
> > +	 * kvm_flush_remote_tlbs().  Although flushing remote TLBs requires all
> > +	 * ASIDs to be flushed, KVM uses a single ASID for L1 and L2, and
> > +	 * unconditionally does a TLB flush on both nested VM-Enter and nested
> > +	 * VM-Exit (via kvm_mmu_reset_context()).
> > +	 */
> > +	svm_flush_tlb_asid(vcpu, is_guest_mode(vcpu));
> >  }
> >  
> >  static void svm_flush_tlb_gva(struct kvm_vcpu *vcpu, gva_t gva)
> > @@ -4075,6 +4081,11 @@ static void svm_flush_tlb_gva(struct kvm_vcpu *vcpu, gva_t gva)
> >  	invlpga(gva, svm_get_current_asid(svm));
> >  }
> >  
> > +static void svm_flush_tlb_guest(struct kvm_vcpu *vcpu)
> > +{
> > +	svm_flush_tlb_asid(vcpu, is_guest_mode(vcpu));
> > +}
> > +
> >  static inline void sync_cr8_to_lapic(struct kvm_vcpu *vcpu)
> >  {
> >  	struct vcpu_svm *svm = to_svm(vcpu);
> > @@ -5187,7 +5198,7 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
> >  	.flush_tlb_all = svm_flush_tlb_all,
> >  	.flush_tlb_current = svm_flush_tlb_current,
> >  	.flush_tlb_gva = svm_flush_tlb_gva,
> > -	.flush_tlb_guest = svm_flush_tlb_asid,
> > +	.flush_tlb_guest = svm_flush_tlb_guest,
> >  
> >  	.vcpu_pre_run = svm_vcpu_pre_run,
> >  	.vcpu_run = svm_vcpu_run,
> 
> 
> Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Thanks!

> 
> 
> Best regards,
> 	Maxim Levitsky
> 
> 
> 
> 
>