Re: [PATCH 1/2] x86/HVM: latch linear->phys translation results

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 09/06/16 13:13, Jan Beulich wrote:
>>>> On 09.06.16 at 13:54, <andrew.cooper3@citrix.com> wrote:
>> On 08/06/16 14:09, Jan Beulich wrote:
>>> ... to avoid re-doing the same translation later again (in a retry, for
>>> example). This doesn't help very often according to my testing, but
>>> it's pretty cheap to have, and will be of further use subsequently.
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>>>
>>> --- a/xen/arch/x86/hvm/emulate.c
>>> +++ b/xen/arch/x86/hvm/emulate.c
>>> @@ -678,6 +678,19 @@ static struct hvm_mmio_cache *hvmemul_fi
>>>      return cache;
>>>  }
>>>  
>>> +static void latch_linear_to_phys(struct hvm_vcpu_io *vio, unsigned long gla,
>>> +                                 unsigned long gpa, bool_t write)
>>> +{
>>> +    if ( vio->mmio_access.gla_valid )
>>> +        return;
>>> +
>>> +    vio->mmio_gva = gla & PAGE_MASK;
>> This suggest that mmio_vga is mis-named.
>>
>> Looking at the uses, handle_mmio_with_translation() is used
>> inconsistently, with virtual addresses from the shadow code, but linear
>> addresses from nested hap code.
>>
>> Clearly one of these two users are buggy for guests running in a non
>> flat way, and it looks to be the shadow side which is buggy.
> Right - this field is certainly meant to be a linear address (as all
> segment information is gone by the time we get here). But I can't
> seem to see an issue with the shadow instance: The "virtual"
> address here is the CR2 value of a #PF, which clearly is a linear
> address.
>
> And anyway all you say is orthogonal to the change here.

Right.  Still, given that there are 4 instances of mmio_gva, renaming it
to mmio_gla for correctness wouldn't be difficult.

That or it can be deferred to a cleanup patch.

Either way, Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

>
>> Following my recent fun with invlpg handling, I am much more conscious
>> about the distinction between virtual and linear addresses.  I wonder if
>> we want to go so far as to have a TYPE_SAFE() for it, to try and avoid
>> further misuse?
> Not sure; if you're up to it, I wouldn't mind, but maybe an
> alternative would be to have a (segment,offset) container
> instead for virtual addresses, such that only linear addresses
> can get passed as plain number?

That is a much better idea.  There is a whole lot of cleanup needed,
relating to this.

~Andrew


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