Re: [kvmarm] [PATCH v5 13/14] KVM: ARM: Handle I/O aborts

Alexander Graf
2013-01-14T18:53:20Z

On 01/14/2013 07:50 PM, Will Deacon wrote:
> On Mon, Jan 14, 2013 at 06:43:19PM +0000, Russell King - ARM Linux wrote:
>> On Mon, Jan 14, 2013 at 01:25:39PM -0500, Christoffer Dall wrote:
>>> However, unifying all instruction decoding within arch/arm is quite
>>> the heavy task, and requires agreeing on some canonical API that
>>> people can live with and it will likely take a long time. I seem to
>>> recall there were also arguments against unifying kprobe code with
>>> other instruction decoding, as the kprobe code was also written to
>>> work highly optimized under certain assumptions, if I understood
>>> previous comments correctly.
>> Yes, I know Rusty had a go.
>>
>> What I think may make sense is to unify this and the alignment code.
>> They're really after the same things, which are:
>>
>> - Given an instruction, and register set, calculate the address of the
>> access, size, number of accesses, and the source/destination registers.
>> - Update the register set as though the instruction had been executed
>> by the CPU.
>>
>> However, I've changed tack slightly from the above in the last 10 minutes
>> or so. I'm thinking a little more that we might be able to take what we
>> already have in alignment.c and provide it with a set of accessors
>> according to size etc.
> FWIW, KVM only needs this code for handling complex MMIO instructions, which
> aren't even generated by recent guest kernels. I'm inclined to suggest removing
> this emulation code from KVM entirely given that it's likely to bitrot as
> it is executed less and less often.

That'd mean that you heavily limit what type of guests you're executing,
which I don't think is a good idea.

Alex