Re: [PATCH v15 01/11] multicall: add no preemption ability between two calls

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 10/09/14 11:07, Jan Beulich wrote:
>>>> On 10.09.14 at 11:43, <andrew.cooper3@citrix.com> wrote:
>> Actually, on further thought, using multicalls like this cannot possibly
>> be correct from a functional point of view.
>>
>> Even with the no preempt flag between a wrmsr/rdmsr hypercall pair,
>> there is no guarantee that accesses to remote cpus msrs won't interleave
>> with a different natural access, clobbering the results of the wrmsr.
>>
>> However this is solved, the wrmsr/rdmsr pair *must* be part of the same
>> synchronous thread of execution on the appropriate cpu.  You can trust
>> that interrupts won't play with these msrs, but you absolutely can't
>> guarantee that IPI/wrmsr/IPI/rdmsr will work.
> Not sure I follow, particularly in the context of the white listing of
> MSRs permitted here (which ought to not include anything the
> hypervisor needs control over).
>
> Jan
>

Consider two dom0 vcpus both using this new multicall mechanism to read
QoS information for different domains, which end up both targeting the
same remote cpu.  They will both end up using IPI/wrmsr/IPI/rdmsr, which
may interleave and clobber the first wrmsr.

~Andrew