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

[Sean Christopherson]

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@xxxxxxxxxx>
> ---
>  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)
  {

  }

And then tlb_tags.min becomes something like tlb_tags.offset (that's a terrible
name, but you probably get the gist).