Re: [PATCH v2 1/3] KVM: x86: assign two bits to track SPTE kinds

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Thu, Dec 12, 2019 at 12:28:27AM +0100, Paolo Bonzini wrote:
> On 11/12/19 20:13, Sean Christopherson wrote:
> > Assuming we haven't missed something, the easiest fix would be to reduce
> > the MMIO generation by one bit and use bits 62:54 for the MMIO generation.
> 
> Yes, and I mistakenly thought it would be done just by adjusting 
> PT64_SECOND_AVAIL_BITS_SHIFT.
> 
> I will test and send formally something like this:
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 6f92b40d798c..aa2d86f42b9a 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -405,11 +405,13 @@ static inline bool is_access_track_spte(u64 spte)
>  }
>  
>  /*
> - * Due to limited space in PTEs, the MMIO generation is a 19 bit subset of
> + * Due to limited space in PTEs, the MMIO generation is a 18 bit subset of
>   * the memslots generation and is derived as follows:
>   *
>   * Bits 0-8 of the MMIO generation are propagated to spte bits 3-11
> - * Bits 9-18 of the MMIO generation are propagated to spte bits 52-61
> + * Bits 9-17 of the MMIO generation are propagated to spte bits 54-62
>   *
> + * We don't use bit 63 to avoid conflicting with the SVE bit in EPT PTEs.
> + *
>   * The KVM_MEMSLOT_GEN_UPDATE_IN_PROGRESS flag is intentionally not included in
>   * the MMIO generation number, as doing so would require stealing a bit from
> @@ -418,15 +418,16 @@ static inline bool is_access_track_spte(u64 spte)
>   * requires a full MMU zap).  The flag is instead explicitly queried when
>   * checking for MMIO spte cache hits.
>   */
> -#define MMIO_SPTE_GEN_MASK		GENMASK_ULL(18, 0)
> +#define MMIO_SPTE_GEN_MASK		GENMASK_ULL(17, 0)
>  
>  #define MMIO_SPTE_GEN_LOW_START		3
>  #define MMIO_SPTE_GEN_LOW_END		11
>  #define MMIO_SPTE_GEN_LOW_MASK		GENMASK_ULL(MMIO_SPTE_GEN_LOW_END, \
>  						    MMIO_SPTE_GEN_LOW_START)
>  
> -#define MMIO_SPTE_GEN_HIGH_START	52
> -#define MMIO_SPTE_GEN_HIGH_END		61
> +/* Leave room for SPTE_SPECIAL_MASK.  */
> +#define MMIO_SPTE_GEN_HIGH_START	PT64_SECOND_AVAIL_BITS_SHIFT

I'd rather have GEN_HIGH_START be an explicit bit number and then add
a BUILD_BUG_ON(GEN_HIGH_START < PT64_SECOND_AVAIL_BITS_SHIFT) to ensure
the MMIO gen doesn't overlap other stuff.  That way we get a build error
if someone changes PT64_SECOND_AVAIL_BITS_SHIFT, otherwise the MMIO gen
will end up who knows where and probably overwrite NX or EPT.SUPPRESS_VE.

> +#define MMIO_SPTE_GEN_HIGH_END		62
>  #define MMIO_SPTE_GEN_HIGH_MASK		GENMASK_ULL(MMIO_SPTE_GEN_HIGH_END, \
>  						    MMIO_SPTE_GEN_HIGH_START)
>  static u64 generation_mmio_spte_mask(u64 gen)
> 
> 
> Paolo
>