Re: [PATCH v6 0/6] x86/xen: Add in-kernel Xen event channel delivery

[Sean Christopherson <seanjc@google.com>]

On Wed, Jan 19, 2022, David Woodhouse wrote:
> On Wed, 2022-01-19 at 18:36 +0100, Paolo Bonzini wrote:
> > On 1/19/22 09:14, David Woodhouse wrote:
> > > > Or do we have explicit other requirement that needs to dirty guest pages
> > > > without vcpu context at all?
> > > 
> > > Delivering interrupts may want to do so. That's the one we hit for
> > > S390, and I only avoided it for Xen event channel delivery on x86 by
> > > declaring that the Xen shared info page is exempt from dirty tracking
> > > and should*always*  be considered dirty.
> > 
> > We also have one that I just found out about in 
> > kvm_hv_invalidate_tsc_page, called from KVM_SET_CLOCK. :/

I think we can fix that usage though:

https://lore.kernel.org/all/YcTpJ369cRBN4W93@google.com

> > So either we have another special case to document for the dirty ring 
> > buffer (and retroactively so, even), or we're in bad need for a solution.
> 
> Seems like adding that warning is having precisely the desired effect :)

The WARN is certainly useful.  Part of me actually likes the restriction of needing
to have a valid vCPU, at least for x86, as there really aren't many legitimate cases
where KVM should be marking memory dirty without a vCPU.