RE: [Xen-devel] [PATCH] Calculate correct instruction length for data-fault VM exits on VT-x systems

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Keir Fraser
> Sent: 28 April 2006 07:03
> To: Khoa Huynh
> Cc: xen-devel
> Subject: Re: [Xen-devel] [PATCH] Calculate correct 
> instruction length for data-fault VM exits on VT-x systems
> 
> 
> On 28 Apr 2006, at 02:52, Khoa Huynh wrote:
> 
> > It should be noted that VMX only uses this instrlen 
> function when the 
> > hypervisor needs the instruction-length info and that info is 
> > undefined in VMCS, e.g., for MMIO instructions.  In other 
> cases where 
> > the instruction-length field is valid in VMCS, the hypervisor 
> > continues to get that info from VMCS (via vmread operation).
> 
> I don't believe we need the instruction-length at all, and I 
> suspect that the decoder could be removed from hvm/svm 
> entirely. There are two broad categories of instruction I'm 
> thinking of:
>   1. Instructions with their own VMEXIT reason code tend to 
> be really simple so we know their length anyway and, if not, 
> the instr-length field should be valid
>   2. For mmio instructions, the emulator can work out the 
> length for itself and increment eip appropriately. There's no 
> need to know the instruction length in advance of invoking 
> the emulator.
> 
> I guess there may be one or two instructions, particularly on 
> AMD, where we aren't feeding the instruction to the mmio 
> emulator and the instruction isn't fixed length, so perhaps 
> we'll need a small decoder in hvm/svm for those. But even if 
> so, it could be much simpler than what is there right now.

Yes, this is correct. There is a specific routine that takes as an
argument which instruction(s) we're looking for, and calculates it's
length, for this purpose [since we do know which instructions we are
looking for]. 

I'll look at your previous suggestion of merging the MMIO emulation into
x86_emulate later on today. We probably do need to sum up the length and
pass it back to the caller - as that code doesn't know how to update the
correct field of the different processor architectures (vmcb vs. vmcs
vs. stack-frame for Para-virtual machine). But it shouldn't be
particularly hard to achieve this. 

--
Mats


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