Re: [PATCH 02/21] KVM: Allow page-sized MMU caches to be initialized with custom 64-bit values

[Binbin Wu]

On 3/5/2024 2:55 PM, Binbin Wu wrote:
> On 2/28/2024 7:20 AM, Paolo Bonzini wrote:
> > From: Sean Christopherson <seanjc@xxxxxxxxxx>
> >
> > Add support to MMU caches for initializing a page with a custom 64-bit
> > value, e.g. to pre-fill an entire page table with non-zero PTE values.
> > The functionality will be used by x86 to support Intel's TDX, which 
> > needs
> > to set bit 63 in all non-present PTEs in order to prevent !PRESENT page
> > faults from getting reflected into the guest (Intel's EPT Violation #VE
> > architecture made the less than brilliant decision of having the per-PTE
> > behavior be opt-out instead of opt-in).
> >
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> > Message-Id: 
> > <5919f685f109a1b0ebc6bd8fc4536ee94bcc172d.1705965635.git.isaku.yamahata@xxxxxxxxx>
> > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > ---
> >   include/linux/kvm_types.h |  1 +
> >   virt/kvm/kvm_main.c       | 16 ++++++++++++++--
> >   2 files changed, 15 insertions(+), 2 deletions(-)
>
> Reviewed-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>
>
> > diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
> > index d93f6522b2c3..827ecc0b7e10 100644
> > --- a/include/linux/kvm_types.h
> > +++ b/include/linux/kvm_types.h
> > @@ -86,6 +86,7 @@ struct gfn_to_pfn_cache {
> >   struct kvm_mmu_memory_cache {
> >       gfp_t gfp_zero;
> >       gfp_t gfp_custom;
> > +    u64 init_value;
> >       struct kmem_cache *kmem_cache;
> >       int capacity;
> >       int nobjs;
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 9c99c9373a3e..c9828feb7a1c 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -401,12 +401,17 @@ static void kvm_flush_shadow_all(struct kvm *kvm)
> >   static inline void *mmu_memory_cache_alloc_obj(struct 
> > kvm_mmu_memory_cache *mc,
> >                              gfp_t gfp_flags)
> >   {
> > +    void *page;
> > +
> >       gfp_flags |= mc->gfp_zero;
> >         if (mc->kmem_cache)
> >           return kmem_cache_alloc(mc->kmem_cache, gfp_flags);
> > -    else
> > -        return (void *)__get_free_page(gfp_flags);
> > +
> > +    page = (void *)__get_free_page(gfp_flags);
> > +    if (page && mc->init_value)
> > +        memset64(page, mc->init_value, PAGE_SIZE / 
> > sizeof(mc->init_value));

Do we need a static_assert() to make sure mc->init_value is 64bit?

> > +    return page;
> >   }
> >     int __kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, 
> > int capacity, int min)
> > @@ -421,6 +426,13 @@ int __kvm_mmu_topup_memory_cache(struct 
> > kvm_mmu_memory_cache *mc, int capacity,
> >           if (WARN_ON_ONCE(!capacity))
> >               return -EIO;
> >   +        /*
> > +         * Custom init values can be used only for page allocations,
> > +         * and obviously conflict with __GFP_ZERO.
> > +         */
> > +        if (WARN_ON_ONCE(mc->init_value && (mc->kmem_cache || 
> > mc->gfp_zero)))
> > +            return -EIO;
> > +
> >           mc->objects = kvmalloc_array(capacity, sizeof(void *), gfp);
> >           if (!mc->objects)
> >               return -ENOMEM;