Re: [Xen-devel] nvmx deadlock with MSR bitmaps

[Jürgen Groß <jgross@xxxxxxxx>]

On 12.03.20 11:56, Roger Pau Monné wrote:
> On Thu, Mar 12, 2020 at 09:59:48AM +0100, Jan Beulich wrote:
> > On 11.03.2020 19:04, Andrew Cooper wrote:
> > > Specifically, this is a switch from an HVM vcpu, to a PV vcpu, where the
> > > mapcache code tries to access the per-domain mappings on the HVM monitor
> > > table.  It ends up trying to recursively acquire the mapcache lock while
> > > trying to walk %cr2 to identify the source of the fault.
> > >
> > > For nvmx->msr_merged, this needs to either be a xenheap page, or a
> > > globally mapped domheap page.  I'll draft a patch in a moment.
> > >
> > > For map_domain_page(), is there anything we can rationally do to assert
> > > that it isn't called in the middle of a context switch?  This is the
> > > kind of thing which needs to blow up reliably in a debug build.
> > Well, it's not inherently unsafe to do, it's just that
> > mapcache_current_vcpu() would need to avoid using current from
> > context_switch()'s call to set_current() through to
> > __context_switch()'s call to write_ptbase(). A possible
> > detection (if we don't want to make the case work) would
> > seem to be ASSERT(current == this_cpu(curr_vcpu)). But of course
> > there's also this extra logic in mapcache_current_vcpu() to deal
> > with a PV vCPU having a null v->arch.guest_table, which I'm once
> > again struggling to see under what conditions it might happen.
> > The Dom0 building case can't be meant with there being
> > mapcache_override_current() on that path. I'm wondering if the
> > comment there is misleading and it's really to cover the case
> > where, coming from a PV vCPU, current was already set to the
> > idle vCPU by context_switch() (which would have a null
> > v->arch.guest_table) - I wouldn't call this "we are running a
> > paravirtualised guest". But in such a case the logic here would
> > simply be a (too) special case of what you're describing as the
> > issue with nVMX.
> Looking at the code in context_switch and __context_switch would it be
> possible to set current to the next vCPU after all the from hooks have
> been called?
>
> Ie: set_current could be moved into __context_switch after the call to
> pd->arch.ctxt_switch->from(p).
No, wouldn't work. When switching to idle __context_switch() is normally
not called in order to avoid switching the address space in case the
same vcpu will be scheduled again after idle. This is the reason why
current and curr_vcpu can be different.

> I'm also not sure I understand the difference between context_switch
> and __context_switch, and how are callers supposed to use them.
That is scheduler black magic. :-)


Juergen

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