Re: [PATCH v5 RESEND 15/17] x86/VPMU: NMI-based VPMU support

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 04/26/2014 04:48 AM, Tian, Kevin wrote:
>> From: Boris Ostrovsky [mailto:boris.ostrovsky@oracle.com]
>> Sent: Wednesday, April 23, 2014 8:51 PM
>>
>> Add support for using NMIs as PMU interrupts.
>>
>> Most of processing is still performed by vpmu_do_interrupt(). However, since
>> certain operations are not NMI-safe we defer them to a softint that
>> vpmu_do_interrupt()
>> will schedule:
>> * For PV guests that would be send_guest_vcpu_virq()
>> * For HVM guests it's VLAPIC accesses and hvm_get_segment_register() (the
>> later
>> can be called in privileged profiling mode when the interrupted guest is an
>> HVM one).
>>
>> With send_guest_vcpu_virq() and hvm_get_segment_register() for PV(H) and
>> vlapic
>> accesses for HVM moved to sofint, the only routines/macros that
>> vpmu_do_interrupt()
>> calls in NMI mode are:
>> * memcpy()
>> * querying domain type (is_XX_domain())
>> * guest_cpu_user_regs()
>> * XLAT_cpu_user_regs()
>> * raise_softirq()
>> * vcpu_vpmu()
>> * vpmu_ops->arch_vpmu_save()
>> * vpmu_ops->do_interrupt() (in the future for PVH support)
>>
>> The latter two only access PMU MSRs with {rd,wr}msrl() (not the _safe
>> versions
>> which would not be NMI-safe).
>>
>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> if a SCHEDULE_SOFTIRQ happens before NMI_SOFTIRQ, will it cause any race
> condition when accessing previous recorded content in nmi softirq handler?

I think you are right.

I in fact had an earlier version where I'd put the record into a linked 
list but when I later looked at that code again I took it out because I 
thought it was unnecessary. But now that you mentioned it I may need to 
resurrect that code. Let me think about this again.

-boris