Re: [PATCH] x86/CPUID: suppress IOMMU related hypervisor leaf data

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.12.2020 11:54, Roger Pau Monné wrote:
> On Mon, Nov 09, 2020 at 11:54:09AM +0100, Jan Beulich wrote:
>> Now that the IOMMU for guests can't be enabled "on demand" anymore,
>> there's also no reason to expose the related CPUID bit "just in case".
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> I'm not sure this is helpful from a guest PoV.
> 
> How does the guest know whether it has pass through devices, and thus
> whether it needs to check if this flag is present or not in order to
> safely pass foreign mapped pages (or grants) to the underlying devices?
> 
> Ie: prior to this change I would just check whether the flag is
> present in CPUID to know whether FreeBSD needs to use a bounce buffer
> in blkback and netback when running as a domU. If this is now
> conditionally set only when the IOMMU is enabled for the guest I
> also need to figure a way to know whether the domU has any passed
> through device or not, which doesn't seem trivial.

I'm afraid I don't understand your concern and/or description of
the scenario. Prior to the change, the bit was set unconditionally.
To me, _that_ was making the bit useless - no point in checking
something which is always set anyway (leaving aside old Xen
versions). I also don't follow how presence / absence of a passed
through device mattering (or not) would have changed with this
patch.

Jan