Re: [Xen-devel] [PATCH v3] nested SVM: adjust guest handling of structure mappings

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.11.13 at 14:16, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> On 11/11/13 12:53, Jan Beulich wrote:
>> +    for ( io_bitmap = hvm_map_guest_frame_ro(gfn, 0); ; )
>> +    {
>> +        enabled = io_bitmap && test_bit(port, io_bitmap);
>> +        if ( !enabled || !--size )
>> +            break;
>> +        if ( unlikely(++port == 8 * PAGE_SIZE) )
>> +        {
>> +            hvm_unmap_guest_frame(io_bitmap, 0);
>> +            io_bitmap = hvm_map_guest_frame_ro(++gfn, 0);
>> +            port -= 8 * PAGE_SIZE;
>> +        }
>>      }
> 
> Ok - this safe now, but I don't understand the reasoning for introducing
> this loop?
> 
> The ioio exit value gives us a single port, and the size of access on
> that specific port.
> 
> The switch statement tells us exactly which gfn the relevant bit refers
> to, surely a single hvm_map_guest_frame_ro() is sufficient?

When the operation spans multiple ports (INW, INL, etc), multiple
bits need to be looked at. And when the access is misaligned and
crosses a 32k (port number) boundary, more than one page needs
looking at.

Jan


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