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SubjectRe: [PATCH 12/13] KVM: x86: add KVM_MEM_X86_SMRAM memory slot flag
On 04/30/2015 02:36 PM, Paolo Bonzini wrote:
> This adds an arch-specific memslot flag that hides slots unless the
> VCPU is in system management mode.
>
> Some care is needed in order to limit the overhead of x86_gfn_to_memslot
> when compared with gfn_to_memslot. Thankfully, we have __gfn_to_memslot
> and search_memslots which are the same, so we can add some extra output
> to search_memslots. The compiler will optimize it as dead code in
> __gfn_to_memslot, and will use it to thread jumps in x86_gfn_to_memslot.
>
>
> struct kvm_memory_slot *x86_gfn_to_memslot(struct kvm_vcpu *vcpu, gfn_t gfn)
> {
> - struct kvm_memory_slot *slot = gfn_to_memslot(vcpu->kvm, gfn);
> + /* By using search_memslots directly the compiler can optimize away
> + * the "if (found)" check below.
> + *
> + * It cannot do the same for gfn_to_memslot because it is not inlined,
> + * and it also cannot do the same for __gfn_to_memslot because the
> + * kernel is compiled with -fno-delete-null-pointer-checks.
> + */
> + bool found;
> + struct kvm_memslots *memslots = kvm_memslots(vcpu->kvm);
> + struct kvm_memory_slot *slot = search_memslots(memslots, gfn, &found);
> +
> + if (found && unlikely(slot->flags & KVM_MEM_X86_SMRAM) && !is_smm(vcpu))
> + return NULL;
>

Alternative approach: store the memslot pointer in the vcpu, instead of
of vcpu->kvm. The uapi interface code can generate two memslot tables
to be stored in kvm->, one for smm and one for !smm.

This is a little more generic and can be used for other asymmetric
memory maps causes (the only one I can think of, mapping the APIC to
different physical addresses, can't be efficiently supported).

> return slot;
> }
> @@ -112,7 +125,15 @@ EXPORT_SYMBOL_GPL(x86_read_guest);
> int x86_gfn_to_hva_cache_init(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
> gpa_t gpa, unsigned long len)
> {
> - return kvm_gfn_to_hva_cache_init(kvm, ghc, gpa, len);
> + int r = kvm_gfn_to_hva_cache_init(kvm, ghc, gpa, len);
> +
> + if (r < 0)
> + return r;
> +
> + /* Use slow path for reads and writes to SMRAM. */
> + if (ghc->memslot && (ghc->memslot->flags & KVM_MEM_X86_SMRAM))
> + ghc->memslot = NULL;
> + return r;
> }
> EXPORT_SYMBOL_GPL(x86_gfn_to_hva_cache_init);
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index e7cfee09970a..ea3db5d89f67 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -800,16 +800,18 @@ static inline void kvm_guest_exit(void)
> * gfn_to_memslot() itself isn't here as an inline because that would
> * bloat other code too much.
> */
> -static inline struct kvm_memory_slot *
> -search_memslots(struct kvm_memslots *slots, gfn_t gfn)
> +static __always_inline struct kvm_memory_slot *
> +search_memslots(struct kvm_memslots *slots, gfn_t gfn, bool *found)
> {
> int start = 0, end = slots->used_slots;
> int slot = atomic_read(&slots->lru_slot);
> struct kvm_memory_slot *memslots = slots->memslots;
>
> if (gfn >= memslots[slot].base_gfn &&
> - gfn < memslots[slot].base_gfn + memslots[slot].npages)
> + gfn < memslots[slot].base_gfn + memslots[slot].npages) {
> + *found = true;
> return &memslots[slot];
> + }
>
> while (start < end) {
> slot = start + (end - start) / 2;
> @@ -823,16 +825,20 @@ search_memslots(struct kvm_memslots *slots, gfn_t gfn)
> if (gfn >= memslots[start].base_gfn &&
> gfn < memslots[start].base_gfn + memslots[start].npages) {
> atomic_set(&slots->lru_slot, start);
> + *found = true;
> return &memslots[start];
> }
>
> + *found = false;
> return NULL;
> }
>
> static inline struct kvm_memory_slot *
> __gfn_to_memslot(struct kvm_memslots *slots, gfn_t gfn)
> {
> - return search_memslots(slots, gfn);
> + bool found;
> +
> + return search_memslots(slots, gfn, &found);
> }
>
> static inline unsigned long
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index e82c46b9860f..b8b76106a003 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -716,6 +716,10 @@ static int check_memory_region_flags(struct kvm_userspace_memory_region *mem)
> #ifdef __KVM_HAVE_READONLY_MEM
> valid_flags |= KVM_MEM_READONLY;
> #endif
> +#ifdef __KVM_ARCH_VALID_FLAGS
> + BUILD_BUG_ON(__KVM_ARCH_VALID_FLAGS & KVM_MEMSLOT_INVALID);
> + valid_flags |= __KVM_ARCH_VALID_FLAGS;
> +#endif
>
> if (mem->flags & ~valid_flags)
> return -EINVAL;



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