[MODERATED] Re: [patch V6 09/14] MDS basics 9

On Fri, Mar 01, 2019 at 10:47:47PM +0100, speck for Thomas Gleixner wrote:
> Subject: [patch V6 09/14] x86/speculation/mds: Conditionally clear CPU buffers on idle entry
> From: Thomas Gleixner <[email protected]>
> 
> Add a static key which controls the invocation of the CPU buffer clear
> mechanism on idle entry. This is independent of other MDS mitigations
> because the idle entry invocation to mitigate the potential leakage due to
> store buffer repartitioning is only necessary on SMT systems.
> 
> Add the actual invocations to the different halt/mwait variants which
> covers all usage sites. mwaitx is not patched as it's not available on
> Intel CPUs.
> 
> The buffer clear is only invoked before entering the C-State to prevent
> that stale data from the idling CPU is spilled to the Hyper-Thread sibling
> after the Store buffer got repartitioned and all entries are available to
> the non idle sibling.
> 
> When coming out of idle the store buffer is partitioned again so each
> sibling has half of it available. Now CPU which returned from idle could be
> speculatively exposed to contents of the sibling, but the buffers are
> flushed either on exit to user space or on VMENTER.
> 
> When later on conditional buffer clearing is implemented on top of this,
> then there is no action required either because before returning to user
> space the context switch will set the condition flag which causes a flush
> on the return to user path.
> 
> Note, that the buffer clearing on idle is only sensible on CPUs which are
> solely affected by MSBDS and not any other variant of MDS because the other
> MDS variants cannot be mitigated when SMT is enabled, so the buffer
> clearing on idle would be a window dressing exercise.
> 
> This intentionally does not handle the case in the acpi/processor_idle
> driver which uses the legacy IO port interface for C-State transitions for
> two reasons:
> 
>  - The acpi/processor_idle driver was replaced by the intel_idle driver
>    almost a decade ago. Anything Nehalem upwards supports it and defaults
>    to that new driver.
> 
>  - The legacy IO port interface is likely to be used on older and therefore
>    unaffected CPUs or on systems which do not receive microcode updates
>    anymore, so there is no point in adding that.
> 
> Signed-off-by: Thomas Gleixner <[email protected]>
> Reviewed-by: Borislav Petkov <[email protected]>
> Reviewed-by: Greg Kroah-Hartman <[email protected]>

Reviewed-by: Frederic Weisbecker <[email protected]>