From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marcelo Tosatti <mtosatti@redhat.com>
Subject: Re: [PATCH v3] KVM: x86 emulator: access GPRs on demand
Date: Thu, 23 Aug 2012 05:14:27 -0300
Message-ID: <20120823081427.GA27248@amt.cnet>
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 <20120817172938.GA21708@amt.cnet>
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Cc: kvm@vger.kernel.org
To: Avi Kivity <avi@redhat.com>
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On Sun, Aug 19, 2012 at 12:32:36PM +0300, Avi Kivity wrote:
> On 08/17/2012 08:29 PM, Marcelo Tosatti wrote:
> > On Thu, Aug 16, 2012 at 05:54:49PM +0300, Avi Kivity wrote:
> >> Instead of populating the the entire register file, read in registers
> >> as they are accessed, and write back only the modified ones.  This
> >> saves a VMREAD and VMWRITE on Intel (for rsp, since it is not usually
> >> used during emulation), and a two 128-byte copies for the registers.
> >> 
> > 
> >> @@ -2715,14 +2764,17 @@ int emulator_task_switch(struct x86_emulate_ctxt *ctxt,
> >>  {
> >>  	int rc;
> >>  
> >> +	invalidate_registers(ctxt);
> >>  	ctxt->_eip = ctxt->eip;
> >>  	ctxt->dst.type = OP_NONE;
> >>  
> >>  	rc = emulator_do_task_switch(ctxt, tss_selector, idt_index, reason,
> >>  				     has_error_code, error_code);
> >>  
> >> -	if (rc == X86EMUL_CONTINUE)
> >> +	if (rc == X86EMUL_CONTINUE) {
> >>  		ctxt->eip = ctxt->_eip;
> >> +		writeback_registers(ctxt);
> >> +	}
> >>  
> >>  	return (rc == X86EMUL_UNHANDLEABLE) ? EMULATION_FAILED : EMULATION_OK;
> >>  }
> > 
> > 
> > No clear point when emulator register cache is active, when it is
> > not (AFAICS this patch does not invalidate registers on emulation start
> > (the above being one of the exceptions) does not clear valid bit on
> > writeback-to-vcpu-cache on emulation exit).
> 
> It is cleared when emulation starts.  For the non-insn-emulation entry
> points, there is an explicit invalidate.  For the emulation entry point,
> there is a memset() that clears everything up to _regs, which includes
> the cache.  This discrepancy isn't nice, but it preexists.  I don't know
> whether we should decompose the memset() or not, it is rather efficient.
> 
> > 
> > Concern is that emulator can start with cached registers marked as valid 
> > but in fact are invalid from previous emulation round.
> > 
> > Maybe move invalidate() to init_emulate_ctxt?
> > 
> 
> See the memset() in init_decode_cache().

Right. Applied, thanks.