linux-kernel - Re: [PATCH] KVM: Veirfy memory slot only for readability

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Date:	Fri, 02 Dec 2011 10:15:54 +0900
From:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>
To:	Sasha Levin <levinsasha928@...il.com>
CC:	linux-kernel@...r.kernel.org, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>, kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: Veirfy memory slot only for readability

(2011/12/02 4:42), Sasha Levin wrote:
> It's enough for memory slot to be readable, as the comment above the check
> states.
> 
> A user should be able to create read-only memory slot.

I submitted the original patch like you to speed up page table walking,
a hot path in KVM, and Avi applied the patch with changing the VERIFY_READ
to VERIFY_WRITE:  on x86, both do the same check.  You can see that on the
commit.

After that, Xiao started to write with __xxx_user() based on this check IIRC.
So you should keep the code as is and change the comment if you like!

Thanks,
	Takuya

> 
> Cc: Avi Kivity<avi@...hat.com>
> Cc: Marcelo Tosatti<mtosatti@...hat.com>
> Cc: kvm@...r.kernel.org
> Signed-off-by: Sasha Levin<levinsasha928@...il.com>
> ---
>   virt/kvm/kvm_main.c |    2 +-
>   1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index e289486..b92883f 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -727,7 +727,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
>   	/* We can read the guest memory with __xxx_user() later on. */
>   	if (user_alloc&&
>   	((mem->userspace_addr&  (PAGE_SIZE - 1)) ||
> -	     !access_ok(VERIFY_WRITE,
> +	     !access_ok(VERIFY_READ,
>   			(void __user *)(unsigned long)mem->userspace_addr,
>   			mem->memory_size)))
>   		goto out;

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