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Re: [PATCH] x86/spec-ctrl: Use IST RSB protection for !SVM systems
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- Date: Mon, 15 Aug 2022 09:33:23 +0000
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- Cc: Roger Pau Monne <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Mon, 15 Aug 2022 09:33:41 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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- Thread-topic: [PATCH] x86/spec-ctrl: Use IST RSB protection for !SVM systems
On 15/08/2022 09:26, Jan Beulich wrote:
> On 05.08.2022 12:38, Andrew Cooper wrote:
>> --- a/xen/arch/x86/spec_ctrl.c
>> +++ b/xen/arch/x86/spec_ctrl.c
>> @@ -1327,8 +1327,24 @@ void __init init_speculation_mitigations(void)
>> * mappings.
>> */
>> if ( opt_rsb_hvm )
>> + {
>> setup_force_cpu_cap(X86_FEATURE_SC_RSB_HVM);
>>
>> + /*
>> + * For SVM, Xen's RSB safety actions are performed before STGI, so
>> + * behave atomically with respect to IST sources.
>> + *
>> + * For VT-x, NMIs are atomic with VMExit (the NMI gets queued but
>> not
>> + * delivered) whereas other IST sources are not atomic.
>> Specifically,
>> + * #MC can hit ahead the RSB safety action in the vmexit path.
>> + *
>> + * Therefore, it is necessary for the IST logic to protect Xen
>> against
>> + * possible rogue RSB speculation.
>> + */
>> + if ( !cpu_has_svm )
>> + default_spec_ctrl_flags |= SCF_ist_rsb;
> Only now, when I'm about to backport this, it occurs to me to ask: Why
> is this !cpu_has_svm rather than cpu_has_vmx?
Because it is only SVM known to be safe.
> Plus shouldn't this further
> be gated upon HVM actually being in use (i.e. in particular CONFIG_HVM=y
> in the first place)?
Perhaps, but not locally here. All of init_speculation_mitigations()
wants reconsidering if you're going down that route.
~Andrew
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