Re: [Xen-devel] [PATCH] SVM: limit GIF=0 region

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 26.06.18 at 12:52, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 26/06/18 10:52, Jan Beulich wrote:
>>>>> On 26.06.18 at 10:45, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> On 26/06/2018 08:32, Jan Beulich wrote:
>>>> Use EFLAGS.IF for all ordinary purposes; there's in particular no need
>>>> to unduly defer NMI/#MC. Clear/set GIF solely around VMRUN itself. This
>>>> has the additional advantage that svm_stgi_label now indeed marks the
>>>> only place where GIF is being set.
>>>>
>>>> A note regarding the main STI placement: Orignally I had it at the place
>>>> the main STGI was sitting at so far. However, my Fam15 box reliably
>>>> locks up hard with this, unless I have the NMI watchdog enabled. I can
>>>> only deduce that the CPU doesn't like STGI with EFLAGS.IF clear plus
>>>> some other condition (the lockup occurs only after exiting the boot
>>>> loader in the guest). As there's nothing wrong with interrupts being on
>>>> right after VMRUN, I've decided to put the STI right after the CLGI
>>>> (matching what KVM does, i.e. having a fair chance of working
>>>> everywhere).
>>> I'd like some input from AMD on this observation, because it is
>>> completely bizarre.
>> Would be nice indeed.
>>
>>>> @@ -96,13 +100,11 @@ __UNLIKELY_END(nsvm_hap)
>>>>          SPEC_CTRL_ENTRY_FROM_HVM    /* Req: b=curr %rsp=regs/cpuinfo, 
>>>> Clob: acd */
>>>>          /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. 
>>>> */
>>>>  
>>>> -        STGI
>>>> -GLOBAL(svm_stgi_label)
>>> Unfortunately, the stgi label must remain here. 
>>> SPEC_CTRL_ENTRY_FROM_HVM depends on NMIs being blocked to avoid
>>> corrupting guest state.
>> I'm afraid I don't follow: How are things safe then without NMIs blocked
>> for VMX?
> 
> 27.5.5 VMExits > Updating Non-Register State
> 
> VM exits caused directly by non-maskable interrupts (NMIs) cause
> blocking by NMI (see Table 24-3). Other VM exits do not affect blocking
> by NMI.

Telling us that an NMI can occur at any point between
vmx_asm_vmexit_handler and the point where guest state was
saved in SPEC_CTRL_ENTRY_FROM_HVM. So afaict if the macro is
indeed unsafe right now, it's the macro that needs to change,
not the patch we're discussing here.

Jan



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