Re: [PATCH] x86/shadow: replace p2m_is_valid() uses

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.03.2025 13:30, Roger Pau Monné wrote:
> On Wed, Mar 12, 2025 at 12:30:40PM +0100, Jan Beulich wrote:
>> The justification for dropping p2m_mmio_dm from p2m_is_valid() was wrong
>> for two of the shadow mode uses.
>>
>> In _sh_propagate() we want to create special L1 entries for p2m_mmio_dm
>> pages. Hence we need to make sure we don't bail early for that type.
>>
>> In _sh_page_fault() we want to handle p2m_mmio_dm by forwarding to
>> (internal or external) emulation. Pull the !p2m_is_mmio() check out of
>> the || expression (as otherwise it would need adding to the lhs as
>> well).
>>
>> In both cases, p2m_is_valid() in combination with p2m_is_grant() still
>> doesn't cover foreign mappings. Hence use p2m_is_any_ram() plus (as
>> necessary) p2m_mmio_* instead.
>>
>> Fixes: be59cceb2dbb ("x86/P2M: don't include MMIO_DM in p2m_is_valid()")
>> Reported-by: Luca Fancellu <Luca.Fancellu@xxxxxxx>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks.

>> @@ -2366,8 +2371,8 @@ static int cf_check sh_page_fault(
>>      gmfn = get_gfn(d, gfn, &p2mt);
>>  
>>      if ( shadow_mode_refcounts(d) &&
>> -         ((!p2m_is_valid(p2mt) && !p2m_is_grant(p2mt)) ||
>> -          (!p2m_is_mmio(p2mt) && !mfn_valid(gmfn))) )
>> +         !p2m_is_mmio(p2mt) &&
>> +         (!p2m_is_any_ram(p2mt) || !mfn_valid(gmfn)) )
> 
> Would you mind adding some comment here about the need to forward
> p2m_mmio_dm to the emulation, and hence not possible to short-circuit
> here?

Will this do?

    /*
     * p2m_mmio_dm in particular is handled further down, and hence can't be
     * short-circuited here. Furthermore, while not fitting with architectural
     * behavior, propagating #PF to the guest when a sensible shadow entry
     * can't be written is necessary. Without doing so (by installing a non-
     * present entry) we'd get back right here immediately afterwards, thus
     * preventing the guest from making further forward progress.
     */

Jan