Re: [edk2-devel] [PATCH v1] OvmfPkg/PlatformInitLib: Don't override user specified PciMmio64Size

["Laszlo Ersek" <lersek@redhat.com>]

On 11/3/23 14:15, Laszlo Ersek wrote:
> On 11/3/23 06:15, Vivek Kasireddy wrote:
>> If the user specified a size for the PCI MMIO window via the option:
>> -fw_cfg name=opt/ovmf/X-PciMmio64Mb,string=32768
>> then this patch ensures that the mmio window is not resized again.
>>
>> Essentially, this prevents the change introduced in the following
>> patch from taking effect:
>> commit ecb778d0ac62560aa172786ba19521f27bc3f650
>> Author: Gerd Hoffmann <kraxel@redhat.com>
>> Date:   Tue Oct 4 15:47:27 2022 +0200
>>
>>     OvmfPkg/PlatformInitLib: dynamic mmio window size
>>
>>     In case we have a reliable PhysMemAddressWidth use that to dynamically
>>     size the 64bit address window.  Allocate 1/8 of the physical address
>>     space and place the window at the upper end of the address space.
>>
>> The problem this patch is trying to solve is the VFIO mapping failures:
>> VFIO_MAP_DMA failed: Invalid argument
>> vfio_dma_map(0x557b2f2736d0, 0x380000000000, 0x1000000, 0x7f98ac400000) = -22 (Invalid argument)
>> that occur when we try to passthrough the graphics device to the guest:
>> qemu-system-x86_64 -m 4096 -enable-kvm -cpu host -smp cores=4,threads=2,sockets=1
>> -device vfio-pci,host=0000:00:02.0 -bios OVMF.fd -nographic
>>
>> The above failures seem to occur because of a mismatch between the
>> PhysMemAddressWidth and the Host IOMMU address width. More specifically,
>> if the PhysMemAddressWidth is bigger than the IOMMU address width,
>> VFIO fails to map the MMIO regions as the IOVAs would be larger
>> than the IOMMU aperture regions. When tested on modern Intel platforms
>> such as ADL, MTL, etc, OVMF determines PhysMemAddressWidth = 46 which
>> matches the Host address width but the IOMMU address width seems to
>> range anywhere from 38 to 48 depending on the IOMMU hardware
>> capabilities, version, etc.
>>
>> One way to address this issue is if we ensure that PhysMemAddressWidth
>> matches IOMMU address width:
>> -cpu host,host-phys-bits=on,host-phys-bits-limit=<IOMMU address width>
>> However, this requires the user to figure out the IOMMU address width;
>> which can be determined by looking at the 16-21 bits of the cap value:
>> cat /sys/devices/virtual/iommu/dmar0/intel-iommu/cap
>> or by reading the DMAR_CAP_REG register. But this does not seem like
>> a reasonable approach to solve this problem.
> 
> Very nice problem description, already outlining the solution as well.
> 
>>
>> Therefore, this problem requires an OVMF specific solution to retain
>> the prior behavior. To this end, this patch reuses the X-PciMmio64Mb
>> option to opt-out of the behavior introduced in the above commit
>> instead of adding a new option or mechanism.
> 
> No, the right solution is to enhance QEMU to query the host IOMMU
> address width. Then the following options arise:
> 
> - either pass *both* the host CPU address width *and* the host IOMMU
> address width down to OVMF, and teach OVMF to pick the stricter
> limitation, for dynamically sizing the MMIO window
> 
> - or let QEMU calulate the stricter width internally, and pass that
> (sole, scalar) piece of information down to OVMF. Teach OVMF to query
> this new piece of information, and size the MMIO window accordingly.
> 
> Basically the QEMU command line-based workaround that you describe is
> what we need to automate (except we need a new information channel for
> it, because presenting the strict host IOMMU address width as the VCPU
> address width (via CPUID) to the guest is smelly).
> 
> I agree that the proposed patch can function as a stop-gap, but the QEMU
> command line hack is already a stop-gap. And for the long term, this
> patch is not good enough. We should enhance the dynamic sizing, now that
> Gerd has put it into place.

... I do agree however that the current behavior is strange -- the user
specifies an explicit fw_cfg knob for OVMF, and OVMF ignores it (for
whatever reason).

I'd like to know what Gerd thinks of this.

Personally I'd ACK the *code* in this patch, just to restore the correct
priority between the dynamic sizing and the explicit fw_cfg knob, if:

(a) the commit message referred to that exactly (i.e., the to the proper
priority between these two configuration avenues), and

(b) there were a promise to enhance QEMU and OVMF as I suggest above.

I don't want the fw_cfg knob to stick around as a permanent excuse for
not improving the dynamic sizing -- now that we *do have* dynamic sizing.

BTW: I'd suggest renaming "QemuFwCfgSizeSpecified" to
"PciMmio64MbOverride" or something like that. The important trait is
that the value is an override (or direct setting) from the user; fw_cfg
is incidental / irrelevant where the value is consumed.

Thanks!
Laszlo



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