linux-kernel - Re: [PATCH 02/11] KVM: guest_memfd: Use AS_INACCESSIBLE when creating guest

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Date: Mon, 29 Apr 2024 15:15:02 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org
Cc: seanjc@...gle.com, michael.roth@....com, isaku.yamahata@...el.com
Subject: Re: [PATCH 02/11] KVM: guest_memfd: Use AS_INACCESSIBLE when creating
 guest_memfd inode

On 4/4/24 8:50 PM, Paolo Bonzini wrote:
> From: Michael Roth <michael.roth@....com>
> 
> truncate_inode_pages_range() may attempt to zero pages before truncating
> them, and this will occur before arch-specific invalidations can be
> triggered via .invalidate_folio/.free_folio hooks via kvm_gmem_aops. For
> AMD SEV-SNP this would result in an RMP #PF being generated by the
> hardware, which is currently treated as fatal (and even if specifically
> allowed for, would not result in anything other than garbage being
> written to guest pages due to encryption). On Intel TDX this would also
> result in undesirable behavior.
> 
> Set the AS_INACCESSIBLE flag to prevent the MM from attempting
> unexpected accesses of this sort during operations like truncation.
> 
> This may also in some cases yield a decent performance improvement for
> guest_memfd userspace implementations that hole-punch ranges immediately
> after private->shared conversions via KVM_SET_MEMORY_ATTRIBUTES, since
> the current implementation of truncate_inode_pages_range() always ends
> up zero'ing an entire 4K range if it is backing by a 2M folio.
> 
> Link: https://lore.kernel.org/lkml/ZR9LYhpxTaTk6PJX@google.com/
> Suggested-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Michael Roth <michael.roth@....com>
> Message-ID: <20240329212444.395559-6-michael.roth@....com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  virt/kvm/guest_memfd.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 0f4e0cf4f158..5a929536ecf2 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -357,6 +357,7 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
>  	inode->i_private = (void *)(unsigned long)flags;
>  	inode->i_op = &kvm_gmem_iops;
>  	inode->i_mapping->a_ops = &kvm_gmem_aops;
> +	inode->i_mapping->flags |= AS_INACCESSIBLE;
>  	inode->i_mode |= S_IFREG;
>  	inode->i_size = size;
>  	mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);