Re: [PATCH 1/2] s390x/pci: add support for guests that request direct mapping

[Matthew Rosato]

On 12/9/24 4:01 PM, David Hildenbrand wrote:
> On 09.12.24 20:29, Matthew Rosato wrote:
> 
> Hi,
> 
> Trying to wrap my head around that ... you mention that "pin the entirety of 
> guest memory".
> 
> Do you mean that we will actually end up longterm pinning all guest RAM in 
> the kernel, similar to what vfio ends up doing?

Yes.  Actually, the usecase here is specifically PCI passthrough via vfio-pci 
on s390.  Unlike other platforms, the default s390 approach only pins on-demand 
and doesn't longterm pin all guest RAM, which is nice from a memory footprint 
perspective but pays a price via all those guest-2 RPCIT instructions.  The 
goal here is now provide the optional alternative to longterm pin like other 
platforms. 

> 
> In that case, it would be incompatible with virtio-balloon (and without 
> modifications with upcoming virtio-mem). Is there already a mechanism in 
> place to handle that -- a call  to ram_block_discard_disable() -- or even a 
> way to support coordinated discarding of RAM (e.g., virtio-mem + vfio)?

Good point, should be calling add ram_block_discard_disable(true) when set 
register + a corresponding (false) during deregister...  Will add for v2.

As for supporting coordinated discard, I was hoping to subsequently look at 
virtio-mem for this.

Thanks,
Matt