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@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; > > > 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 -- மணிவண்ணன் சதாசிவம்
