Re: [RFC PATCH v2 04/25] KVM: x86/mmu: Support specifying a minimum TLB tag

[Sean Christopherson <seanjc@google.com>]

On Thu, Jul 23, 2026, Yosry Ahmed wrote:
> On Wed, Jul 22, 2026 at 05:28:00PM -0700, Sean Christopherson wrote:
> > On Tue, Jun 16, 2026, Yosry Ahmed wrote:
> > > In preparation for using the TLB tags allocator for SVM, which has a
> > > range of ASIDs allocated for SEV/SNP, pass in a minimum TLB tag when
> > > initializing the TLB tags allocator. The bitmap is conceptually shifted
> > > such that bit=0 corresponds to tag=min.
> > > 
> > > Specifying the minimum value during initialization also makes the API
> > > clearer, as the passed number of tags becomes the actual number of
> > > *usable* tags, and tag=0 is explicitly excluded by the caller.
> > > 
> > > No functional change intended for VMX as VPID=0 is not used anyway.
> > > 
> > > Signed-off-by: Yosry Ahmed <yosry@kernel.org>
> > > ---
> > >  arch/x86/kvm/mmu.h     |  2 +-
> > >  arch/x86/kvm/mmu/mmu.c | 38 +++++++++++++++++++++++---------------
> > >  arch/x86/kvm/vmx/vmx.h |  3 ++-
> > >  3 files changed, 26 insertions(+), 17 deletions(-)
> > > 
> > > diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> > > index 9a2916012cbff..cfffee92b8b71 100644
> > > --- a/arch/x86/kvm/mmu.h
> > > +++ b/arch/x86/kvm/mmu.h
> > > @@ -337,7 +337,7 @@ static inline bool kvm_is_gfn_alias(struct kvm *kvm, gfn_t gfn)
> > >  
> > >  typedef unsigned int kvm_tlb_tag_t;
> > >  
> > > -int kvm_init_tlb_tags(unsigned int nr);
> > > +int kvm_init_tlb_tags(kvm_tlb_tag_t min, unsigned int nr);
> > >  void kvm_destroy_tlb_tags(void);
> > >  kvm_tlb_tag_t kvm_alloc_tlb_tag(void);
> > >  void kvm_free_tlb_tag(kvm_tlb_tag_t tag);
> > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > index e021ed562502f..bf2e0c2205631 100644
> > > --- a/arch/x86/kvm/mmu/mmu.c
> > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > @@ -8197,24 +8197,26 @@ static struct {
> > >  	spinlock_t	lock;
> > >  	unsigned long	*bitmap;
> > >  	unsigned int	nr;
> > > +	kvm_tlb_tag_t	min;
> > >  } tlb_tags;
> > >  
> > > -int kvm_init_tlb_tags(unsigned int nr)
> > > +int kvm_init_tlb_tags(kvm_tlb_tag_t min, unsigned int nr)
> > 
> > Hmm, I think this code should handle "nr - min" instead of requiring the caller
> > to do so.  Because people that know how SEV+ works, but aren't familiar with this
> > code (like I am right now), will probably find it confusing that nothing in here
> > appears to account for the "reserved" ASIDs.
> > 
> > Maybe this?  With documentation that reserved ASIDs start at 0, inclusive, and
> > are contiguous.
> > 
> >   int kvm_init_tlb_tags(unsigned int nr, unsigned nr_reserved)
> >   {
> > 
> >   }
> 
> So essentially make the reserved tags included in 'nr', and min becomes
> nr_reserved. I wanted to keep it generic by not factoring in the assumptions
> about the reserved tags, 

Eh, by having a @min you've already lost that battle though.  It's only "generic"
in the sense that it obfuscates why there is a minimum in the first place.  E.g.
if SEV+ used ASIDs starting that the top and counting backwards, taking only the
min wouldn't suffice.

> but we can revisit later if ever needed (very unlikely).