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Subject: Re: [PATCH 0/5] Introduce support for PSF mitigation
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org
References: <20210406155004.230790-1-rsaripal@amd.com>
 <20210407223905.5dqgzdaibvgq7jeq@treble>
From:   "Saripalli, RK" <rsaripal@amd.com>
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Date:   Thu, 8 Apr 2021 09:56:47 -0500
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Josh, thank you for taking the time to review the patches.

On 4/7/2021 5:39 PM, Josh Poimboeuf wrote:
> On Tue, Apr 06, 2021 at 10:49:59AM -0500, Ramakrishna Saripalli wrote:
>> Because PSF speculation is limited to the current program context,
>> the impact of bad PSF speculation is very similar to that of
>> Speculative Store Bypass (Spectre v4)
>>
>> Predictive Store Forwarding controls:
>> There are two hardware control bits which influence the PSF feature:
>> - MSR 48h bit 2 – Speculative Store Bypass (SSBD)
>> - MSR 48h bit 7 – Predictive Store Forwarding Disable (PSFD)
>>
>> The PSF feature is disabled if either of these bits are set.  These bits
>> are controllable on a per-thread basis in an SMT system. By default, both
>> SSBD and PSFD are 0 meaning that the speculation features are enabled.
>>
>> While the SSBD bit disables PSF and speculative store bypass, PSFD only
>> disables PSF.
>>
>> PSFD may be desirable for software which is concerned with the
>> speculative behavior of PSF but desires a smaller performance impact than
>> setting SSBD.
> 
> Hi Ramakrishna,
> 
> Is there a realistic scenario where an application would want to disable
> PSF, but not disable SSB?

It is possible most applications have been reviewed and scrubbed for SSB-type attacks but PSF-type issues may not have been looked at yet.
This may be one of the cases where SSB is enabled but PSF is disabled until the application(s) are scrubbed for the same.

In certain cases, disabling PSF may have less impact performance-wise than disabling SSB.


> 
> Maybe I'm missing something, but I'd presume an application would either
> care about this class of attacks, or not.
>