Re: [RFC PATCH] PCI: Add quirk to always map ivshmem as write-back

[Borislav Petkov <bp@alien8.de>]

On Thu, Jun 12, 2025 at 06:22:33PM +1000, Alexey Kardashevskiy wrote:
> QEMU Inter-VM Shared Memory (ivshmem) is designed to share a memory
> region between guest and host. The host creates a file, passes it to QEMU
> which it presents to the guest via PCI BAR#2. The guest userspace
> can map /sys/bus/pci/devices/0000:01:02.3/resource2(_wc) to use the region
> without having the guest driver for the device at all.
> 
> The problem with this, since it is a PCI resource, the PCI sysfs
> reasonably enforces:

Ok, so I read it up until now and can't continue because all I hear is a big
honking HACK alarm here!

Shared memory which is presented to a guest via PCI BAR?!?

Can it get any more ugly than this?

I hope I'm missing an important aspect here...

> diff --git a/drivers/pci/mmap.c b/drivers/pci/mmap.c
> index 8da3347a95c4..8495bee08fae 100644
> --- a/drivers/pci/mmap.c
> +++ b/drivers/pci/mmap.c
> @@ -35,6 +35,7 @@ int pci_mmap_resource_range(struct pci_dev *pdev, int bar,
>  	if (write_combine)
>  		vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
>  	else
> +	else if (!(pci_resource_flags(pdev, bar) & IORESOURCE_CACHEABLE))
	^^^^^^

This can't build.

-- 
Regards/Gruss,
    Boris.

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