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);
--
2.49.0
