Re: [PATCH v3 04/27] KVM: x86/mmu: Support specifying reserved TLB tags

[Jim Mattson]

On Mon, Oct 12, 2026 at 4:36 PM Yosry Ahmed <yosry@xxxxxxxxxx> wrote:
>
> In preparation for using the TLB tags allocator for SVM, which has a
> range of ASIDs allocated for SEV/SNP, pass in the number of reserved TLB
> tags when initializing the TLB tags allocator. The bitmap is
> conceptually shifted such that bit=0 corresponds to tag=nr_reserved.
>
> Specifying the minimum value during initialization also makes the API
> clearer, as tag=0 is now explicitly reserved by the caller.
>
> No functional change intended for VMX as VPID=0 was already implicitly
> reserved.
>
> Signed-off-by: Yosry Ahmed <yosry@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu.h     |  2 +-
>  arch/x86/kvm/mmu/mmu.c | 42 ++++++++++++++++++++++++++++--------------
>  arch/x86/kvm/vmx/vmx.h |  2 +-
>  3 files changed, 30 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index de79e002edf8f..b443a5083bfb0 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -413,7 +413,7 @@ 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(unsigned int nr_total, unsigned int nr_reserved);
>  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 97d49247d9d82..c096c1a2b4684 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -8182,9 +8182,17 @@ static struct {
>         spinlock_t      lock;
>         unsigned long   *bitmap;
>         unsigned int    nr;
> +       kvm_tlb_tag_t   start;
>  } tlb_tags;
>
> -int kvm_init_tlb_tags(unsigned int nr)
> +/*
> + * Initialize @nr_total TLB tags, where @nr_reserved of those tags are reserved.
> + * Reserved tags start at 0 and are contiguous. Hence, usable tags are
> + * [nr_reserved, nr_total). At least one reserved tag is required, as 0 is the
> + * host TLB tag on both VMX and AMD, and is the value returned on a failed tag

Nit: VMX and SVM -or- Intel and AMD

> + * allocation.
> + */
> +int kvm_init_tlb_tags(unsigned int nr_total, unsigned int nr_reserved)
>  {
>         /*
>          * Limit the number of TLB tags to VMX's hardcoded maximum of 0x10000
> @@ -8194,7 +8202,12 @@ int kvm_init_tlb_tags(unsigned int nr)
>          * need a solution that works for both Intel and AMD.
>          */
>         const unsigned int MAX_NR_TLB_TAGS = VMX_NR_VPIDS;
> +       unsigned int nr;
>
> +       if (WARN_ON_ONCE(!nr_reserved || nr_reserved > nr_total))
> +               return -EINVAL;
> +
> +       nr = nr_total - nr_reserved;
>         if (!nr)
>                 return 0;
>
> @@ -8208,13 +8221,8 @@ int kvm_init_tlb_tags(unsigned int nr)
>         if (!tlb_tags.bitmap)
>                 return -ENOMEM;
>
> -       /*
> -        * 0 is the host's TLB tag for both VMX's VPID and SVM's ASID, and is
> -        * returned on failed allocations (e.g. no more tags left).
> -        */
> -       __set_bit(0, tlb_tags.bitmap);
> -
>         tlb_tags.nr = nr;
> +       tlb_tags.start = nr_reserved;

Can you also clear tlb_tags.start in kvm_destroy_tlb_tags()?