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SubjectRe: [PATCH RESEND] x86/speculation: Fix user-mode spectre-v2 protection with KERNEL_IBRS
On Mon, Feb 20, 2023 at 11:30:46PM +0000, Andrew Cooper wrote:
> 1) Intel's legacy IBRS
> 2) AMD's regular IBRS

Yeah, we don't do that in the kernel.

> 3) AMD's AutoIBRS
> which all have different relevant behaviours for userspace.  Just so
> it's written out coherently in at least one place...
> When SEV-SNP is enabled in firmware, whether or not it's being used by
> software, AutoIBRS keeps indirect predictions inhibited in all of
> ASID0.  That's all host userspace to the non-hypervisor devs reading
> this thread.


> For any AMD configuration setting STIBP, there must be an IBPB after
> having set STIBP.   Setting STIBP alone does not evict previously
> created shared predictions.  This one can go subtly wrong for anyone who
> assumes that Intel STIBP and AMD STIBP have the same behaviour.

We will IBPB eventually... on the next context switch.

> Furthermore, extra care needs taking on vmexit because transitioning
> from the guest STIBP setting to the host STIBP setting can leave shared
> predictions intact.

From what I can tell from looking at the SVM code, we don't do anything
special there when restoring SPEC_CTRL.


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