Re: [PATCH hci-8.0 0/2] pci: batch memory transactions around mapping updates #VSTOR-129348

[Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>]

Ack
Applied to hci-8.0

On 9/10/26 11:32 PM, Denis V. Lunev wrote:
> Restoring a PCI device's config space remaps its BARs, updates the bridge
> windows when it is a bridge, and toggles the bus master region. Each of
> those ends a memory transaction of its own, and a transaction commit
> re-renders the flatviews it touches, at a cost which grows with the number
> of memory regions in the machine.
> 
> So a machine with a rich PCIe topology pays that render many times over
> while an incoming migration stream is read, once or more per device, even
> though nothing between the steps looks at an address space.
> 
> Both patches wrap an existing sequence in a single transaction. There is
> no new API and no behaviour a guest can observe: each transaction opens and
> closes inside the function that starts it, so nothing sees an intermediate
> state which was not already reachable. pci_bridge_update_mappings() has
> worked this way for the bridge windows for a long time; the first patch
> just makes pci_update_mappings() consistent with it.
> 
> Measured on a Windows Server 2022 guest with 64 vCPUs and 65 pcie-root-ports,
> the non-iterable part of an incoming migration drops by about a third. The
> saving scales with the number of PCI devices, so it is small for a plain
> machine and large for the topologies which hurt today.
> 
> Note that pci_update_mappings() is also reached from the runtime config
> write path, not only from migration. Several BAR moves in one config write
> now share a rebuild instead of taking one each, which is the same win in
> miniature and equally invisible to the guest.
> 
> CC: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Denis V. Lunev <den@virtuozzo.com>
> 
> Denis V. Lunev (2):
>   pci: batch BAR remapping into one memory transaction #VSTOR-129348
>   pci: load a device's config inside one memory transaction
>     #VSTOR-129348
> 
>  hw/pci/pci.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>