Hi Eric,
> -----Original Message-----
> From: Eric Auger <[email protected]>
> Sent: 20 October 2025 17:44
> To: Shameer Kolothum <[email protected]>; qemu-
> [email protected]; [email protected]
> Cc: [email protected]; Jason Gunthorpe <[email protected]>; Nicolin
> Chen <[email protected]>; [email protected]; [email protected];
> Nathan Chen <[email protected]>; Matt Ochs <[email protected]>;
> [email protected]; [email protected];
> [email protected]; [email protected];
> [email protected]; [email protected]; [email protected];
> [email protected]
> Subject: Re: [PATCH v4 12/27] hw/arm/smmuv3-accel: Make use of
> get_msi_address_space() callback
>
> External email: Use caution opening links or attachments
>
>
> Hi Shameer,
>
> On 9/29/25 3:36 PM, Shameer Kolothum wrote:
> > Here we return the IOMMU address space if the device has S1 translation
> > enabled by Guest. Otherwise return system address space.
> >
> > Signed-off-by: Shameer Kolothum
> <[email protected]>
> > Signed-off-by: Shameer Kolothum <[email protected]>
> > ---
> > hw/arm/smmuv3-accel.c | 21 +++++++++++++++++++++
> > 1 file changed, 21 insertions(+)
> >
> > diff --git a/hw/arm/smmuv3-accel.c b/hw/arm/smmuv3-accel.c
> > index 790887ac31..f4e01fba6d 100644
> > --- a/hw/arm/smmuv3-accel.c
> > +++ b/hw/arm/smmuv3-accel.c
> > @@ -387,6 +387,26 @@ static void
> smmuv3_accel_unset_iommu_device(PCIBus *bus, void *opaque,
> > }
> > }
> >
> > +static AddressSpace *smmuv3_accel_find_msi_as(PCIBus *bus, void
> *opaque,
> > + int devfn)
> > +{
> > + SMMUState *bs = opaque;
> > + SMMUPciBus *sbus = smmu_get_sbus(bs, bus);
> > + SMMUv3AccelDevice *accel_dev = smmuv3_accel_get_dev(bs, sbus,
> bus, devfn);
> > + SMMUDevice *sdev = &accel_dev->sdev;
> > +
> > + /*
> > + * If the assigned vfio-pci dev has S1 translation enabled by
> > + * Guest, return IOMMU address space for MSI translation.
> > + * Otherwise, return system address space.
> > + */
> > + if (accel_dev->s1_hwpt) {
> > + return &sdev->as;
> > + } else {
> > + return &address_space_memory;
> > + }
> At the moment I don't understand this code either. In case of emulated
> device it then returns address_space_memory whereas I would have
> expected the opposite. I definitively need to trace things ;-)
We have,
[VIRT_GIC_ITS] = { 0x08080000, 0x00020000 },
I added a few prints in kvm_arch_fixup_msi_route() so that it may help
to understand how the translation of MSI doorbell is performed here.
If we return IOMMU addr space(&sdev->as) here,
kvm_arch_fixup_msi_route: MSI IOVA=0xffbf0040 msi_addr_lo=0xffbf0040
msi_addr_hi=0x0
kvm_arch_fixup_msi_route: Translated doorbell_gpa= 0x8090040
kvm_arch_fixup_msi_route: ret:MSI IOVA=0xffbf0040 translated:
msi_addr_lo=0x8090040 msi_addr_hi=0x0
It gets the correct vITS gpA address after the translation through
address_space_translate().
Since host uses the (MSI_IOVA_BASE, MSI_IOVA_LENGTH) for ITS doorbell mapping
and using IORT RMR we make sure there is an identity mapping for that range, it
all
works fine.
Now, suppose if we return system addr space(&address_space_memory):
kvm_arch_fixup_msi_route: MSI IOVA=0xffbf0040 msi_addr_lo 0xffbf0040
msi_addr_hi 0x0
kvm_arch_fixup_msi_route: address_space_memory, nothing to do, return
And the device doorbell gets configured with gIOVA 0xffbf0040 instead of the
vITS gPA
as Nicolin explained in the other thread.
Hope this helps.
Thanks,
Shameer
