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

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Message-ID: <opdpelyb26bzp723lyxljjb2dmxgunkcjlvpkxgbrxaxhoycv6@eigu7etse3g7>
Date: Wed, 25 Jun 2025 10:28:29 -0600
From: Manivannan Sadhasivam <mani@...nel.org>
To: Alexey Kardashevskiy <aik@....com>
Cc: linux-pci@...r.kernel.org, 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

On Tue, Jun 24, 2025 at 11:42:47AM +1000, Alexey Kardashevskiy wrote:
> 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.
> > > 

It just makes me nervous to change the sematics of the sysfs attribute, even if
the user knows what it is expecting. Now the "resourceN_wc" essentially becomes
"resourceN_wb", which goes against the rule of sysfs I'm afraid.

> > > 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;
> > > 

I guess this would the cleanest approach. The old tools can continue to suffer
from the performance issue and the new tools can work more faster.

- Mani

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