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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.03.2020 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).
> 
> I'm also not sure I understand the difference between context_switch
> and __context_switch, and how are callers supposed to use them.

The latter does the full switch, while the former only records
that a switch is pending, and does the parts that can't be
deferred. For this reason set_current() can't be moved to
__context_switch() without applying extreme care, I'm afraid.

Jan

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