Re: [PATCH 0/8] Add support for pvpanic mmio device

[Mihai Carabas]

On 10/22/2020 1:17 PM, Peter Maydell wrote:
> On Thu, 22 Oct 2020 at 09:25, Mihai Carabas <mihai.carabas@oracle.com> wrote:
> > The patchset was assembled from chuncks from some old patches from 2018 [1]
> > which were left unmerged and some additions from me. Surprisingly their Linux
> > kernel counterpart were merged (so the pvpanic driver from the kernel supports
> > mmio).
> >
> > I have seen the discussions about moving the pvpanic to PCI [1]. Those patches
> > were sent but nothing happened. Also they are not trivial and require major
> > modifications at the driver level also. Given the fact that we already have
> > mmio driver support for pvpanic in the Linux kernel, I have sent these patches
> > to ask again the maintainers if this can be merged.
>
> I'm afraid the answer is still the same. You need to provide
> a convincing argument for why this needs to be an MMIO
> device rather than a PCI device. I really don't want to
> add MMIO devices to the virt board if I can avoid it,
> because they're all extra code and potential extra
> security boundary attack surface. PCI devices are guest
> probeable and user-pluggable so they're almost always
> nicer to use than MMIO.

Thank you for your input.

The reason why pvpanic should be MMIO is that is a special device which 
is not used commonly by the user (aka VM), it is not need to be 
hot-plugable and it does not have a hardware correspondent to be a PCI 
device. Another reason is that MMIO support was accepted in the kernel 
driver and it is pretty useless there without a device.

I know it seems that I want to get this on the short-path, but at this 
point having a kernel driver in the upstream and no device to test it 
against it is pretty weird.

Thank you,
Mihai