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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 12:53:08 +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 11:53:16 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
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()
~Andrew
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