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Message-ID: <0be55c001aa1a538a02055aa244c655262228ce4.camel@intel.com>
Date: Thu, 26 Jan 2023 21:37:23 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "zhi.wang.linux@...il.com" <zhi.wang.linux@...il.com>,
"Christopherson,, Sean" <seanjc@...gle.com>
CC: "sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"Shahar, Sagi" <sagis@...gle.com>,
"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
"Aktas, Erdem" <erdemaktas@...gle.com>,
"dmatlack@...gle.com" <dmatlack@...gle.com>,
"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
"Yamahata, Isaku" <isaku.yamahata@...el.com>,
"pbonzini@...hat.com" <pbonzini@...hat.com>
Subject: Re: [PATCH v11 030/113] KVM: x86/mmu: Replace hardcoded value 0 for
the initial value for SPTE
On Wed, 2023-01-25 at 17:22 +0000, Sean Christopherson wrote:
> On Wed, Jan 25, 2023, Zhi Wang wrote:
> > On Thu, 12 Jan 2023 08:31:38 -0800
> > isaku.yamahata@...el.com wrote:
> >
> > This refactor patch is quite hacky.
> >
> > Why not change the purpose of vcpu->arch.mmu_shadow_page.gfp_zero and let the
> > callers respect that the initial value of spte can be configurable? It will be
> > generic and not TDX-specific, then kvm_init_shadow_page() is not required,
> > mmu_topup_shadow_page_cache() can be left un-touched as the refactor can cover
> > other architectures.
> >
> > 1) Let it store the expected nonpresent value and rename it to nonpresent_spte.
>
>
> I agree that handling this in the common code would be cleaner, but repurposing
> gfp_zero gets kludgy because it would require a magic value to say "don't initialize
> the data", e.g. x86's mmu_shadowed_info_cache isn't pre-filled.
>
> And supporting a custom 64-bit init value for kmem_cache-backed caches would require
> restricting such caches to be a multiple of 8 bytes in size.
>
> How about this? Lightly tested.
>
> From: Sean Christopherson <seanjc@...gle.com>
> Date: Wed, 25 Jan 2023 16:55:01 +0000
> Subject: [PATCH] KVM: Allow page-sized MMU caches to be initialized with
> custom 64-bit values
>
> 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@...gle.com>
> ---
> 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 76de36e56cdf..67972db17b55 100644
> --- a/include/linux/kvm_types.h
> +++ b/include/linux/kvm_types.h
> @@ -94,6 +94,7 @@ struct kvm_mmu_memory_cache {
> int nobjs;
> gfp_t gfp_zero;
> gfp_t gfp_custom;
> + u64 init_value;
> struct kmem_cache *kmem_cache;
> int capacity;
> void **objects;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index d255964ec331..78f1e49179a7 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -380,12 +380,17 @@ 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));
> + return page;
> }
>
> int __kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int capacity, int min)
> @@ -400,6 +405,13 @@ 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(sizeof(void *), capacity, gfp);
> if (!mc->objects)
> return -ENOMEM;
>
> base-commit: 503f0315c97739d3f8e645c500d81757dfbf76be
init_value and gfp_zone is kinda redundant. How about removing gfp_zero
completely?
mmu_memory_cache_alloc_obj(...)
{
...
if (!mc->init_value)
gfp_flags |= __GFP_ZERO;
...
}
And in kvm_mmu_create() you initialize all caches' init_value explicitly.
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