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xen-devel
RE: [Xen-devel] [PATCH] Calculate correct instruction length for data-fa
> -----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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