Re: [bugzilla-daemon@kernel.org: [Bug 219619] New: vfio-pci: screen graphics artifacts after 6.12 kernel upgrade]

[Precific <precification@posteo.de>]

On 31.12.24 02:27, Alex Williamson wrote:
> On Mon, 30 Dec 2024 21:03:30 +0000
> Precific <precification@posteo.de> wrote:
> 
>> In my case, commenting out (1) the huge_fault callback assignment from
>> f9e54c3a2f5b suffices for GPU initialization in the guest, even if (2)
>> the 'install everything' loop is still removed.
>>
>> I have uploaded host kernel logs with vfio-pci-core debugging enabled
>> (one log with stock sources, one large log with vfio-pci-core's
>> huge_fault handler patched out):
>> https://bugzilla.kernel.org/show_bug.cgi?id=219619#c1
>> I'm not sure if the logs of handled faults say much about what
>> specifically goes wrong here, though.
>>
>> The dmesg portion attached to my mail is of a Linux guest failing to
>> initialize the GPU (BAR 0 size 16GB with 12GB of VRAM).
> 
> Thanks for the logs with debugging enabled.  Would you be able to
> repeat the test with QEMU 9.2?  There's a patch in there that aligns
> the mmaps, which should avoid mixing 1G and 2MB pages for huge faults.
> With this you should only see order 18 mappings for BAR0.
> 
> Also, in a different direction, it would be interesting to run tests
> disabling 1G huge pages and 2MB huge pages independently.  The
> following would disable 1G pages:
> 
> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> index 1ab58da9f38a..dd3b748f9d33 100644
> --- a/drivers/vfio/pci/vfio_pci_core.c
> +++ b/drivers/vfio/pci/vfio_pci_core.c
> @@ -1684,7 +1684,7 @@ static vm_fault_t vfio_pci_mmap_huge_fault(struct vm_fault *vmf,
>   							     PFN_DEV), false);
>   		break;
>   #endif
> -#ifdef CONFIG_ARCH_SUPPORTS_PUD_PFNMAP
> +#if 0
>   	case PUD_ORDER:
>   		ret = vmf_insert_pfn_pud(vmf, __pfn_to_pfn_t(pfn + pgoff,
>   							     PFN_DEV), false);
> 
> This should disable 2M pages:
> 
> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> index 1ab58da9f38a..d7dd359e19bb 100644
> --- a/drivers/vfio/pci/vfio_pci_core.c
> +++ b/drivers/vfio/pci/vfio_pci_core.c
> @@ -1678,7 +1678,7 @@ static vm_fault_t vfio_pci_mmap_huge_fault(struct vm_fault *vmf,
>   	case 0:
>   		ret = vmf_insert_pfn(vma, vmf->address, pfn + pgoff);
>   		break;
> -#ifdef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP
> +#if 0
>   	case PMD_ORDER:
>   		ret = vmf_insert_pfn_pmd(vmf, __pfn_to_pfn_t(pfn + pgoff,
>   							     PFN_DEV), false);
> 
> And applying both together should be functionally equivalent to
> pre-v6.12.  Thanks,
> 
> Alex
> 

Logs with QEMU 9.1.2 vs. 9.2.0, all huge_page sizes/1G only/2M only: 
https://bugzilla.kernel.org/show_bug.cgi?id=219619#c3

You're right, I was still using QEMU 9.1.2. With 9.2.0, the 
passed-through GPU works fine indeed with both Linux and Windows guests.

The huge_fault calls are aligned nicely with QEMU 9.2.0. Only the lower 
16MB of BAR 0 see repeated calls at 2M/4K page sizes but no misalignment.
The QEMU 9.1.2 'stock' log shows a misalignment with 1G faults (order 
18), e.g., huge_faulting 0x40000 pages at page offset 0 and later 
0x4000. I'm not sure if that is a problem, or if the offsets are simply 
masked off to the correct alignment.
QEMU 9.1.2 also works with 1G pages disabled. Perhaps coincidentally, 
the offsets are aligned properly for order 9 (0x200 'page offset' 
increments) from what I've seen.

Thanks,
Precific