Re: SMMU Stage 2 translation in QEMU

[shashi.mallela@linaro.org]

Hi Eric,
On Mon, 2021-09-13 at 10:19 +0200, Eric Auger wrote:
> Hi Shashi,
> 
> On 9/10/21 3:32 PM, shashi.mallela@linaro.org wrote:
> > So that would be the driver code running in guest OS because i see
> > tables being setup by arm-smmu driver code in linux,which is
> > similar to
> > what happens with ITS(table base addresses programmed in registers
> > by
> > linux driver).
> Stage2 SMMU tables are generally not used today. Only stage 1 is
> used,
> even with VFIO (with KVM).
The stage 2 SMMU capability is required for qemu sbsa reference
platform to satisfy the sbsa level 3 acs(arm compliance suite)
requirements.

> SMMUv3 upstream driver does not support S1+S2 (nested). It only
> supports
> either S1 or S2. Enabling S2 can be done through VFIO driver, if you
> select the VFIO_TYPE1_NESTING_IOMMU IOMMU type. This then calls IOMMU
> .enable_nesting  = arm_smmu_enable_nesting which sets smmu_domain-
> >stage
> = ARM_SMMU_DOMAIN_NESTED. But the name is misleading as it actually
> forces the use of S2 instead of S1.
> 
> However if you look at QEMU VFIO code, no one uses
> VFIO_TYPE1_NESTING_IOMMU.
Since the current smmuv3 implementation in qemu advertises only stage 1
support,from the qemu device point of view is the stage 2 support
included in the integration effort you mentioned or does it need to be
taken up from scratch?
> 
> Note I have worked on 2 stage integration for years, without much
> success yet:
> [RFC v9 00/29] vSMMUv3/pSMMUv3 2 stage VFIO integration
> 
> [PATCH v15 00/12] SMMUv3 Nested Stage Setup (IOMMU part)
> [PATCH v13 00/13] SMMUv3 Nested Stage Setup (VFIO part)
> 
> 
> Thanks
> 
> Eric
> > On Fri, 2021-09-10 at 13:54 +0100, Peter Maydell wrote:
> > > On Fri, 10 Sept 2021 at 13:39, <shashi.mallela@linaro.org> wrote:
> > > > I am referring to the latter,"purely emulated QEMU with an
> > > > emulated
> > > > SMMU that handles accesses to emulated devices"
> > > In that case, the stage 2 tables are set up by the guest
> > > code (running at emulated EL2), just as they would be if
> > > it were running on real hardware.
> > > 
> > > -- PMM