linux-kernel - Re: [RFC 21/33] KVM: Pass memory attribute array as a MMU notifier argument

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Message-ID: <ZUvAnqAIsUP4Vd0J@google.com>
Date:   Wed, 8 Nov 2023 09:08:46 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Nicolas Saenz Julienne <nsaenz@...zon.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hyperv@...r.kernel.org, pbonzini@...hat.com,
        vkuznets@...hat.com, anelkz@...zon.com, graf@...zon.com,
        dwmw@...zon.co.uk, jgowans@...zon.com, corbert@....net,
        kys@...rosoft.com, haiyangz@...rosoft.com, decui@...rosoft.com,
        x86@...nel.org, linux-doc@...r.kernel.org
Subject: Re: [RFC 21/33] KVM: Pass memory attribute array as a MMU notifier argument

On Wed, Nov 08, 2023, Nicolas Saenz Julienne wrote:
> Pass the memory attribute array through struct kvm_mmu_notifier_arg and
> use it in kvm_arch_post_set_memory_attributes() instead of defaulting on
> kvm->mem_attr_array.
> 
> Signed-off-by: Nicolas Saenz Julienne <nsaenz@...zon.com>
> ---
>  arch/x86/kvm/mmu/mmu.c   | 8 ++++----
>  include/linux/kvm_host.h | 5 ++++-
>  virt/kvm/kvm_main.c      | 1 +
>  3 files changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index c0fd3afd6be5..c2bec2be2ba9 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -7311,6 +7311,7 @@ static bool hugepage_has_attrs(struct xarray *mem_attr_array,
>  bool kvm_arch_post_set_memory_attributes(struct kvm *kvm,
>  					 struct kvm_gfn_range *range)
>  {
> +	struct xarray *mem_attr_array = range->arg.mem_attr_array;
>  	unsigned long attrs = range->arg.attributes;
>  	struct kvm_memory_slot *slot = range->slot;
>  	int level;
> @@ -7344,8 +7345,8 @@ bool kvm_arch_post_set_memory_attributes(struct kvm *kvm,
>  			 * misaligned address regardless of memory attributes.
>  			 */
>  			if (gfn >= slot->base_gfn) {
> -				if (hugepage_has_attrs(&kvm->mem_attr_array,
> -						       slot, gfn, level, attrs))
> +				if (hugepage_has_attrs(mem_attr_array, slot,
> +						       gfn, level, attrs))

This is wildly broken.  The hugepage tracking is per VM, whereas the attributes
here are per-VTL.  I.e. KVM will (dis)allow hugepages based on whatever VTL last
changed its protections.