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

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Message-ID: <930fc54c-a88c-49b3-a1a7-6ad9228d84ac@amd.com>
Date: Tue, 24 Jun 2025 11:42:47 +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>, Santosh Shukla <santosh.shukla@....com>,
 "Nikunj A. Dadhania" <nikunj@....com>,
 "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [RFC PATCH] PCI: Add quirk to always map ivshmem as write-back

Ping? Thanks,


On 12/6/25 18:27, Alexey Kardashevskiy wrote:
> 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@....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);
> 

-- 
Alexey