linux-kernel - Re: [PATCH 1/2] KVM: X86: remove read buffer for mmio read

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Message-ID: <4FFABD59.2070108@redhat.com>
Date:	Mon, 09 Jul 2012 14:15:37 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 1/2] KVM: X86: remove read buffer for mmio read

On 07/09/2012 12:02 PM, Xiao Guangrong wrote:
> After commit f78146b0f9230765c6315b2e14f56112513389ad:
> 
>  KVM: Fix page-crossing MMIO
> 
>     MMIO that are split across a page boundary are currently broken - the
>     code does not expect to be aborted by the exit to userspace for the
>     first MMIO fragment.
> 
>     This patch fixes the problem by generalizing the current code for handling
>     16-byte MMIOs to handle a number of "fragments", and changes the MMIO
>     code to create those fragments.
> 
>     Signed-off-by: Avi Kivity <avi@...hat.com>
>     Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>
> 
> Multiple MMIO reads can be merged into mmio_fragments, the read buffer is not
> needed anymore
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
> ---
>  arch/x86/include/asm/kvm_emulate.h |    1 -
>  arch/x86/kvm/emulate.c             |   43 ++++-------------------------------
>  arch/x86/kvm/x86.c                 |    2 -
>  3 files changed, 5 insertions(+), 41 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
> index 1ac46c22..339d7c6 100644
> --- a/arch/x86/include/asm/kvm_emulate.h
> +++ b/arch/x86/include/asm/kvm_emulate.h
> @@ -286,7 +286,6 @@ struct x86_emulate_ctxt {
>  	struct operand *memopp;
>  	struct fetch_cache fetch;
>  	struct read_cache io_read;
> -	struct read_cache mem_read;
>  };
> 

Suppose we have a RMW instruction.  On the first entry to
x86_emulate_insn() we'll drop to userspace and perform the read, and the
seconds we'll read from the cache and complete the write.

Without the read cache this cannot work.

kvm-unit-tests.git has a test for xchg to mmio.  Does it still work?

I agree this code has to go, but it needs to be replaced by something.
Maybe a .valid flag in struct operand.

-- 
error compiling committee.c: too many arguments to function


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