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SubjectRe: [PATCH] vfio/pci: Virtualize Maximum Payload Size
Hi Alex,

On 19/09/2017 22:20, Alex Williamson wrote:
> [cc +linux-pci, Sinan]
> On Tue, 19 Sep 2017 19:50:37 +0200
> Auger Eric <> wrote:
>> Hi Alex,
>> On 19/09/2017 18:58, Alex Williamson wrote:
>>> With virtual PCI-Express chipsets, we now see userspace/guest drivers
>>> trying to match the physical MPS setting to a virtual downstream port.
>>> Of course a lone physical device surrounded by virtual interconnects
>>> cannot make a correct decision for a proper MPS setting. Instead,
>>> let's virtualize the MPS control register so that writes through to
>>> hardware are disallowed. Userspace drivers like QEMU assume they can
>>> write anything to the device and we'll filter out anything dangerous.
>>> Since mismatched MPS can lead to AER and other faults, let's add it
>>> to the kernel side rather than relying on userspace virtualization to
>>> handle it.
>>> Signed-off-by: Alex Williamson <>
>>> ---
>>> Do we have any reason to suspect that a userspace driver has any
>>> dependencies on the physical MPS setting or is this only tuning the
>>> protocol layer and it's transparent to the driver? Note that per the
>>> PCI spec, a device supporting only 128B MPS can hardwire the control
>>> register to 000b, but it doesn't seem PCIe compliant to hardwire it to
>>> any given value, such as would be the appearance if we exposed this as
>>> a read-only register rather than virtualizing it. QEMU would then be
>>> responsible for virtualizing it, which makes coordinating the upgrade
>>> troublesome.
>>> drivers/vfio/pci/vfio_pci_config.c | 6 ++++--
>>> 1 file changed, 4 insertions(+), 2 deletions(-)
>>> diff --git a/drivers/vfio/pci/vfio_pci_config.c b/drivers/vfio/pci/vfio_pci_config.c
>>> index 5628fe114347..91335e6de88a 100644
>>> --- a/drivers/vfio/pci/vfio_pci_config.c
>>> +++ b/drivers/vfio/pci/vfio_pci_config.c
>>> @@ -849,11 +849,13 @@ static int __init init_pci_cap_exp_perm(struct perm_bits *perm)
>>> /*
>>> * Allow writes to device control fields, except devctl_phantom,
>>> - * which could confuse IOMMU, and the ARI bit in devctl2, which
>>> + * which could confuse IOMMU, MPS, which can break communication
>>> + * with other physical devices, and the ARI bit in devctl2, which
>>> * is set at probe time. FLR gets virtualized via our writefn.
>>> */
>>> p_setw(perm, PCI_EXP_DEVCTL,
>> Is it correct that the read value still will be the one written by the
>> guest?
> If we don't do this, the register is read-only, which is what I'm
> suggesting in my comment above doesn't seem spec compliant. If the
> kernel makes it read-only, then userspace (ex. QEMU) would need to
> virtualize it, and we get into a more complicated, two part fix.
>> I see the MMRS can take the read MPS value in some pcie_bus_config
>> values. So a consequence could be that the applied MMRS (which is not
>> virtualized) is lower than what is set by host, due to a guest pcie root
>> port MPSS for instance.
>> So if the above is not totally wrong, shouldn't we virtualize MMRS as well?
> I think MPSS is Maximum Payload Size Supported and MMRS... did you mean
> MRRS (Maximum Read Request Size)?
Yes sorry for the typo. Indeed I meant MRRS.
> My impression is that MRRS is predominantly device and driver
> dependent, not topology dependent. A device can send a read request
> with a size larger than MPS, which implies that the device supplying
> the read data would split it into multiple TLPs based on MPS.
I read that too on the net. However in in (3.0. Nov 10), Rules
for SW Configuration it is written:
"Software must set Max_Read_Request_Size of an isochronous-configured
device with a value that does not exceed the Max_Payload_Size set for
the device."

But on the the other hand some drivers are setting the MMRS directly
without further checking the MPS?
> implementation note in PCIe rev3.1 sec 7.8.4 suggests there may be QoS
> implications of MRRS such that if MRRS>MPS, then the device requesting
> the data may use fewer tokens for the request. I have no idea how we
> could infer any sort of QoS policy though and I don't see that
> in-kernel drivers are bound to any sort of policy.
> The pcie_write_mrrs() function also implies a protocol (which I can't
> find a spec reference to) where it will re-try writing MRRS if the
> value written doesn't stick (and generate a dev_warn on each
> iteration). Unless we want extra warnings in VMs, that suggests we
> don't want this to be read-only.
> So I think we need to allow MRRS writes through to hardware, but I do
> question whether we need to fill the gap that a VM might casually write
> MRRS to a value less than the physical MPS. This is what we'd expect
> today where the virtual topology has an MPS of 128B regardless of the
> physical topology. Do we virtualize MRRS writes such that an MRRS
> value less the physical MPS value never reaches hardware? Can we
> assume that's always invalid? Thanks,


> Alex

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