Re: [PATCH 12/13] KVM: x86: add KVM_MEM_X86_SMRAM memory slot flag

[Avi Kivity <avi.kivity@gmail.com>]

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;