linux-kernel - Re: [PATCH 2/9] KVM: x86: simplify read

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Message-ID: <20120720105840.GA16859@amt.cnet>
Date:	Fri, 20 Jul 2012 07:58:40 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	Avi Kivity <avi@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
	KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 2/9] KVM: x86: simplify read_emulated

On Fri, Jul 20, 2012 at 10:17:36AM +0800, Xiao Guangrong wrote:
> On 07/20/2012 07:58 AM, Marcelo Tosatti wrote:
> 
> >> -	}
> >> +	rc = ctxt->ops->read_emulated(ctxt, addr, mc->data + mc->end, size,
> >> +				      &ctxt->exception);
> >> +	if (rc != X86EMUL_CONTINUE)
> >> +		return rc;
> >> +
> >> +	mc->end += size;
> >> +
> >> +read_cached:
> >> +	memcpy(dest, mc->data + mc->pos, size);
> > 
> > What prevents read_emulated(size > 8) call, with
> > mc->pos == (mc->end - 8) now?
> 
> Marcelo,
> 
> The splitting has been done in emulator_read_write_onepage:
> 
> 	while (bytes) {
> 		unsigned now = min(bytes, 8U);
> 
> 		frag = &vcpu->mmio_fragments[vcpu->mmio_nr_fragments++];
> 		frag->gpa = gpa;
> 		frag->data = val;
> 		frag->len = now;
> 		frag->write_readonly_mem = (ret == -EPERM);
> 
> 		gpa += now;
> 		val += now;
> 		bytes -= now;
> 	}
> 
> So i think it is safe to remove the splitting in read_emulated.

Yes, it is fine to remove it.

But splitting in emulate.c prevented the case of _cache read_ with size
> 8 beyond end of mc->data. Must handle that case in read_emulated.

"What prevents read_emulated(size > 8) call, with mc->pos == (mc->end - 8) now?"


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