linux-kernel - Re: [PATCH 1/2] x86: cpu/bugs: add support for AMD ERAPS feature

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Message-ID: <4b23d73d450d284bbefc4f23d8a7f0798517e24e.camel@amd.com>
Date: Mon, 4 Nov 2024 08:58:41 +0000
From: "Shah, Amit" <Amit.Shah@....com>
To: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "dave.hansen@...el.com"
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Subject: Re: [PATCH 1/2] x86: cpu/bugs: add support for AMD ERAPS feature

On Thu, 2024-10-31 at 16:11 -0700, Dave Hansen wrote:
> On 10/31/24 08:39, Amit Shah wrote:
> ...
> > With the Enhanced Return Address Prediction Security feature,  any
> > hardware TLB flush results in flushing of the RSB (aka RAP in AMD
> > spec).
> > This guarantees an RSB flush across context switches. 
> 
> Check out the APM, volume 2: "5.5.1 Process Context Identifier"
> 
> 	... when system software switches address spaces (by writing
> ...
> 	CR3[62:12]), the processor may use TLB mappings previously
> 	stored for that address space and PCID, providing that bit
> 63 of
> 	the source operand is set to 1.
> 
> tl;dr: PCIDs mean you don't necessarily flush the TLB on context
> switches.

Right - thanks, I'll have to reword that to say the RSB is flushed
along with the TLB - so any action that causes the TLB to be flushed
will also cause the RSB to be flushed.