Re: [Xen-devel] [PATCH] x86/HVM: honor p2m_ram_ro in hvm_map_guest_frame_rw()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.07.15 at 12:26, <wei.liu2@xxxxxxxxxx> wrote:
> On Fri, Jul 24, 2015 at 03:41:26AM -0600, Jan Beulich wrote:
>> @@ -3618,7 +3618,12 @@ static void *__hvm_map_guest_frame(unsig
>>      }
>>  
>>      if ( writable )
> 
> I don't claim I know this piece of code, but checking the pointer but
> not the content looks suspicious.
> 
>> -        paging_mark_dirty(d, page_to_mfn(page));
>> +    {
>> +        if ( !p2m_is_discard_write(p2mt) )
>> +            paging_mark_dirty(d, page_to_mfn(page));
>> +        else
>> +            *writable = 0;
> 
> You then set *writable here, which makes it even more suspicious.

Why? A caller _wanting_ a writable mapping passes non-NULL as
the pointer argument (pre-initialized to point to a variable holding
TRUE aka 1). Upon return the variable will have got set to FALSE
aka 0 if the page shouldn't be written to.

Jan


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