Re: [Xen-devel] [PATCH] x86/HVM: adjust hvm_interrupt_blocked()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 31.08.2023 12:57, Roger Pau Monné wrote:
> On Thu, Aug 31, 2023 at 12:42:58PM +0200, Roger Pau Monné wrote:
>> On Fri, Oct 12, 2018 at 09:58:46AM -0600, Jan Beulich wrote:
>>> First of all, hvm_intsrc_mce was not considered here at all, yet nothing
>>> blocks #MC (other than an already in-progress #MC, but dealing with this
>>> is not the purpose of this patch).
>>>
>>> Additionally STI-shadow only blocks maskable interrupts, but not NMI.
>>
>> I've found the Table 25-3 on Intel SDM vol3 quite helpful:
>>
>> "Execution of STI with RFLAGS.IF = 0 blocks maskable interrupts on the
>> instruction boundary following its execution.1 Setting this bit
>> indicates that this blocking is in effect."
>>
>> And:
>>
>> "Execution of a MOV to SS or a POP to SS blocks or suppresses certain
>> debug exceptions as well as interrupts (maskable and nonmaskable) on
>> the instruction boundary following its execution."
>>
>> Might be worth adding to the commit message IMO.
> 
> So I've found a further footnote that contains:
> 
> "Nonmaskable interrupts and system-management interrupts may also be
> inhibited on the instruction boundary following such an execution of
> STI."
> 
> So we want to take the more restrictive implementation of STI-shadow,
> and block #NMI there also.

Hmm, that text says "may", not will, and imo STI affecting NMI can at best
be viewed as a quirk (quite possibly intentional, for simplifying some
internal logic on the processor). Plus I'm not convinced AMD allows similar
leeway in SVM; at least I can't spot any similar statement in their PM.

Jan