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

[Alexey Kardashevskiy]

Wrong email for Nikunj :) And I missed the KVM ml. Sorry for the noise.


On 12/6/25 18:22, 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:
> - no caching when mapped via "resourceN" (PTE::PCD on x86) or
> - write-through when mapped via "resourceN_wc" (PTE::PWT on x86).
>
> As the result, the host writes are seen by the guest immediately
> (as the region is just a mapped file) but it takes quite some time for
> the host to see non-cached guest writes.
>
> Add a quirk to always map ivshmem's BAR2 as cacheable (==write-back) as
> ivshmem is backed by RAM anyway.
> (Re)use already defined but not used IORESOURCE_CACHEABLE flag.
>
> This does not affect other ways of mapping a PCI BAR, a driver can use
> memremap() for this functionality.
>
> Signed-off-by: Alexey Kardashevskiy <aik@xxxxxxx>
> ---
>
> What is this IORESOURCE_CACHEABLE for actually?
>
> Anyway, the alternatives are:
>
> 1. add a new node in sysfs - "resourceN_wb" - for mapping as writeback
> but this requires changing existing (and likely old) userspace tools;
>
> 2. fix the kernel to strictly follow /proc/mtrr (now it is rather
> a recommendation) but Documentation/arch/x86/mtrr.rst says it is replaced
> with PAT which does not seem to allow overriding caching for specific
> devices (==MMIO ranges).
>
> ---
>   drivers/pci/mmap.c   | 6 ++++++
>   drivers/pci/quirks.c | 8 ++++++++
>   2 files changed, 14 insertions(+)
>
> 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))
>   		vma->vm_page_prot = pgprot_device(vma->vm_page_prot);
>   
>   	if (mmap_state == pci_mmap_io) {
> @@ -46,6 +47,11 @@ int pci_mmap_resource_range(struct pci_dev *pdev, int bar,
>   
>   	vma->vm_ops = &pci_phys_vm_ops;
>   
> +	if (pci_resource_flags(pdev, bar) & IORESOURCE_CACHEABLE)
> +		return remap_pfn_range_notrack(vma, vma->vm_start, vma->vm_pgoff,
> +					       vma->vm_end - vma->vm_start,
> +					       vma->vm_page_prot);
> +
>   	return io_remap_pfn_range(vma, vma->vm_start, vma->vm_pgoff,
>   				  vma->vm_end - vma->vm_start,
>   				  vma->vm_page_prot);
> diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c
> index d7f4ee634263..858869ec6612 100644
> --- a/drivers/pci/quirks.c
> +++ b/drivers/pci/quirks.c
> @@ -6335,3 +6335,11 @@ static void pci_mask_replay_timer_timeout(struct pci_dev *pdev)
>   DECLARE_PCI_FIXUP_FINAL(PCI_VENDOR_ID_GLI, 0x9750, pci_mask_replay_timer_timeout);
>   DECLARE_PCI_FIXUP_FINAL(PCI_VENDOR_ID_GLI, 0x9755, pci_mask_replay_timer_timeout);
>   #endif
> +
> +static void pci_ivshmem_writeback(struct pci_dev *dev)
> +{
> +	struct resource *r = &dev->resource[2];
> +
> +	r->flags |= IORESOURCE_CACHEABLE;
> +}
> +DECLARE_PCI_FIXUP_FINAL(PCI_VENDOR_ID_REDHAT_QUMRANET, 0x1110, pci_ivshmem_writeback);

--
Alexey