On 3/19/25 7:34 PM, Nicolin Chen wrote:
> On Wed, Mar 19, 2025 at 07:09:33PM +0100, Eric Auger wrote:
>>> Option means something like this:
>>> -device smmuv3,accel=on
>>> instead of
>>> -device "smmuv3-accel"
>>> ?
>>>
>>> Yea, I think that's good.
>> Yeah actually that's a big debate for not much. From an implementation
>> pov that shall not change much. The only doubt I have is if we need to
>> conditionnaly expose the MSI RESV regions it is easier to do if we
>> detect we have a smmuv3-accel. what the option allows is the auto mode.
> Mind elaborating your doubt about the MSI RESV region?
>
> Do you mean how VMS code should tag "accel=on" option and generate
> RMR nodes in the IORT table?
yes that was my point. Earlier we detected whether a "nested-smmu" was
part of the object hierarchy. Now we do the same with smmu type and
check if accel=on. I guess we can retrieve the property value but this
is worth to test.
>
>>> We certainly can't do case (a): not all TLBI commands gives an "SID"
>>> field (so would have to broadcast, i.e. underlying SMMU HW would run
>>> commands that were supposed for emulated devices only); in case of
>>> vCMDQ, commands for emulated devices would be issued to real HW and
>> I am still confused about that. For instance if the guest sends an
>> NH_ASID, NH_VA invalidation and it happens both the emulated device and
>> VFIO-device share the same cd.asid (same guest iommu domain, which
>> practically should not happen) why shouldn't we propagate the
>> invalidation to the host. Does the problem come from the usage of vCMDQ
>> or would you foresee the same problem with a generic physical SMMU?
> Host (HW) would end up with executing commands that were issued for
> emulated devices, which impacts performance.
>
> With vCMDQ, QEMU cannot trap command queue because all invalidation
> commands will be issued to HW directly from the guest kernel driver.
> This includes TLBI and ATC_INV commands. It's probably okay to run
> TLBI commands with vCMDQ (again perf impact), while ATC_INV commands
> would result in "unkonwn SID" errors or directly ATC_INV timeouts.
OK understood. Thanks and sorry for the misunderstanding
Eric
>
> Thanks
> Nicolin
>
