On Tue, Feb 25, 2020 at 06:57:18AM -0500, Michael S. Tsirkin wrote: > On Tue, Feb 25, 2020 at 09:24:34AM +0000, Stefan Hajnoczi wrote: > > On Mon, Feb 24, 2020 at 12:10:25PM -0500, Michael S. Tsirkin wrote: > > > On Sat, Feb 22, 2020 at 08:19:16PM +0000, Stefan Hajnoczi wrote: > > > > The KVM_IOREGIONFD_POSTED_WRITES flag > > > > skips waiting for an acknowledgement on write accesses. This is > > > > suitable for accesses that do not require synchronous emulation, such as > > > > doorbell register writes. > > > > > > I would avoid hacks like this until we understand this better. > > > Specificlly one needs to be very careful since memory ordering semantics > > > can differ between a write into an uncacheable range and host writes into > > > a data structure. Reads from one region are also assumed to be ordered with > > > writes to another region, and drivers are known to make assumptions > > > like this. > > > > > > Memory ordering being what it is, this isn't a field I'd be comfortable > > > device writes know what they are doing. > > > > Unlike PCI Posted Writes the idea is not to let the write operations sit > > in a cache. They will be sent immediately just like ioeventfd is > > signalled immediately before re-entering the guest. > > But ioeventfd sits in the cache: the internal counter. The fact it's > signalled does not force a barrier on the signalling thread. It looks > like the same happens here: value is saved with the file descriptor, > other accesses of the same device can bypass the write. See below. > > The purpose of this feature is to let the device emulation program > > handle these writes asynchronously (without holding up the vCPU for a > > response from the device emulation program) but the order of > > reads/writes remains unchanged. > > > > Stefan > > I don't see how this can be implemented without guest changes though. > For example, how do you make sure two writes to such regions are ordered > just like they are on PCI? Register both regions with the same fd. This way the write accesses will be read out by the device emulation program in serialized order. The purpose of the region_id field is to allow device emulation programs that register multiple regions with the same fd to identify which one is being accessed. QEMU would only need 1 fd. Each VFIO userspace device (muser) would need 1 fd. Stefan
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