Re: [PATCH] x86/CPUID: unconditionally set XEN_HVM_CPUID_IOMMU_MAPPINGS

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Jan 18, 2021 at 05:48:37PM +0000, Andrew Cooper wrote:
> On 18/01/2021 17:10, Roger Pau Monné wrote:
> > On Mon, Jan 18, 2021 at 05:04:19PM +0100, Jan Beulich wrote:
> >> On 18.01.2021 16:54, Roger Pau Monné wrote:
> >>> On Mon, Jan 18, 2021 at 12:05:12PM +0100, Jan Beulich wrote:
> >>>> On 15.01.2021 16:01, Roger Pau Monne wrote:
> >>>>> --- a/xen/arch/x86/traps.c
> >>>>> +++ b/xen/arch/x86/traps.c
> >>>>> @@ -1049,11 +1049,10 @@ void cpuid_hypervisor_leaves(const struct vcpu 
> >>>>> *v, uint32_t leaf,
> >>>>>              res->a |= XEN_HVM_CPUID_X2APIC_VIRT;
> >>>>>  
> >>>>>          /*
> >>>>> -         * Indicate that memory mapped from other domains (either 
> >>>>> grants or
> >>>>> -         * foreign pages) has valid IOMMU entries.
> >>>>> +         * Unconditionally set the flag to indicate this version of 
> >>>>> Xen has
> >>>>> +         * been fixed to create IOMMU mappings for grant/foreign maps.
> >>>>>           */
> >>>>> -        if ( is_iommu_enabled(d) )
> >>>>> -            res->a |= XEN_HVM_CPUID_IOMMU_MAPPINGS;
> >>>>> +        res->a |= XEN_HVM_CPUID_IOMMU_MAPPINGS;
> >>>> ... try to clarify the "Unconditionally" here?
> >>> I guess Unconditionally doesn't make much sense, so would be better to
> >>> start the sentence with 'Set ...' instead?
> >> Hmm, this would further move us away from the goal of the comment
> >> making sufficiently clear that a conditional shouldn't be (re-)
> >> introduced here, I would think. Since I can't seem to think of a
> >> good way to express this more briefly than in the description,
> >> and if maybe you can't either, perhaps there's no choice then to
> >> leave it as is, hoping that people would look at the commit before
> >> proposing a further change here.
> > /*
> >  * Unconditionally set the flag to indicate this version of Xen has
> >  * been fixed to create IOMMU mappings for grant/foreign maps.
> >  *
> >  * NB: this flag shouldn't be made conditional on IOMMU presence, as
> >  * it could force guests to resort to using bounce buffers when using
> >  * grant/foreign maps with devices.
> >  */
> >
> > Would be better? (albeit too verbose maybe).
> 
> The comment should be rather more direct.
> 
> 1) Xen 4.10 and older was broken WRT grant maps requesting a DMA
> mapping, and forgot to honour the guest's request.
> 2) 4.11 (and presumably backports) fixed the bug, so the map hypercall
> actually did what the guest asked.
> 3) To work around the bug, guests must bounce buffer all DMA, because it
> doesn't know whether the DMA is originating from an emulated or a real
> device.
> 4) This flag tells guests it is safe not to bounce-buffer all DMA to
> work around the bug.

/*
 * Old versions of Xen are broken when creating grant/foreign maps,
 * and will never create IOMMU entries for such mappings. This was
 * fixed in later versions of Xen, but guests wanting to work on
 * unpatched versions will need to use a bounce buffer in order to
 * avoid sending grant/foreign maps to devices. Whether such bounce
 * buffer mechanism is not needed is indicated by the presence of the
 * following CPUID flag.
 */

Does that seem better?

Thanks, Roger.