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Re: [Xen-devel] [PATCH] SVM: limit GIF=0 region
- To: Jan Beulich <JBeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 26 Jun 2018 12:56:09 +0100
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- Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
- Delivery-date: Tue, 26 Jun 2018 11:56:32 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 26/06/18 12:53, Andrew Cooper wrote:
> On 26/06/18 12:50, Jan Beulich wrote:
>>>>> 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.
> No... It tells you the exact opposite.
>
> NMIs are not delivered while the NMI shadow is in effect, and is why we
> deliberately execute an IRET in the NMI path in vmx_vmexit_handler()
Actually never mind. I'm being very dumb... Let me experiment
~Andrew
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