Re: PCIe atomics in pcie-root-port

[Robin Voetter <robin@streamhpc.com>]

On 4/6/23 20:40, Alex Williamson wrote:

> I think the typical approach for QEMU would be expose options in the
> downstream ports that would then need to be enabled by the user or
> management tool, but that's where the complication begins.  At some
> point we would want management tools to "do the right thing"
> themselves.  Is support for PCIe atomics pervasive enough to default to
> enabling support?

Apparently this is supported from Haswell or newer on Intel, and 
Ryzen/Threadripper/Epyc on AMD hardware. I don't have any official data 
for this, though.

> How do we handle hotplugged endpoints where the
> interconnects do not expose atomics support, or perhaps when they
> expose support that doesn't exist?  At some point in the future when we
> have migration of devices with atomics, do we need to test all endpoint
> to endpoint paths for equivalent atomic support on the migration target?
> Are there capabilities on the endpoint that we can virtualize to
> disable use of atomics if the host and guest topologies are out of sync?
It seems automatic detection is a lot of added complication for this 
feature. For the time being, I think its best to allow enabling PCIe 
atomics using a device property for the pcie-root-port that is disabled 
by default. If general hardware support is good enough, it can be 
enabled by default. Compatibility with older QEMU versions can then be 
added using a newer hw compat in hw/core/machine.c.

Kind regards,

Robin