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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <20250612082233.3008318-1-aik@amd.com>
Date: Thu, 12 Jun 2025 18:22:33 +1000
From: Alexey Kardashevskiy <aik@....com>
To: <linux-pci@...r.kernel.org>
CC: <linux-kernel@...r.kernel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
	"David Woodhouse" <dwmw@...zon.co.uk>, Kai-Heng Feng
	<kai.heng.feng@...onical.com>, Paolo Bonzini <pbonzini@...hat.com>, Sean
 Christopherson <seanjc@...gle.com>, Nikunj A Dadhania
	<nikunj@...ux.vnet.ibm.com>, Santosh Shukla <santosh.shukla@....com>, Alexey
 Kardashevskiy <aik@....com>
Subject: [RFC PATCH] PCI: Add quirk to always map ivshmem as write-back

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@....com>
---

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