Re: [PATCH v19 048/130] KVM: Allow page-sized MMU caches to be initialized with custom 64-bit values

[Isaku Yamahata]

On Tue, Mar 26, 2024 at 11:53:02PM +0800,
Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx> wrote:

> 
> 
> On 2/26/2024 4:25 PM, isaku.yamahata@xxxxxxxxx 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>
> > ---
> >   include/linux/kvm_types.h |  1 +
> >   virt/kvm/kvm_main.c       | 16 ++++++++++++++--
> >   2 files changed, 15 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
> > index 9d1f7835d8c1..60c8d5c9eab9 100644
> > --- a/include/linux/kvm_types.h
> > +++ b/include/linux/kvm_types.h
> > @@ -94,6 +94,7 @@ 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 de38f308738e..d399009ef1d7 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?

I don't see much value.  Is your concern sizeof() part?
If so, we can replace it with 8.

        memset64(page, mc->init_value, PAGE_SIZE / 8);
-- 
Isaku Yamahata <isaku.yamahata@xxxxxxxxx>