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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Mar 12, 2020 at 12:12:12PM +0100, Jürgen Groß wrote:
> 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.

Since the idle vCPU context switch is already a special case we could
maybe place the call to set_current in such special handling, while
leaving the call for the rest of vCPUs in __context_switch after the
->from hooks have been executed?

> > I'm also not sure I understand the difference between context_switch
> > and __context_switch, and how are callers supposed to use them.

Jan points out that __context_switch signals a pending context switch,
in which case my proposal to use set_current might not be suitable, as
we would be changing current without actually doing the context
switch?

I also wonder why __context_switch then needs to call the ->from hook
just to signal a pending context switch. It seems like
__context_switch does a lot of work just to signal a context switch,
which will then be redone when context_switch is actually called?

Thanks, Roger.

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