Re: [PATCH 1/5] KVM: rseq: Update rseq when processing NOTIFY_RESUME on xfer to KVM guest

[Sean Christopherson <seanjc@google.com>]

On Fri, Aug 20, 2021, Mathieu Desnoyers wrote:
> Without the lazy clear scheme, a rseq c.s. would look like:
> 
>  *                     init(rseq_cs)
>  *                     cpu = TLS->rseq::cpu_id_start
>  *   [1]               TLS->rseq::rseq_cs = rseq_cs
>  *   [start_ip]        ----------------------------
>  *   [2]               if (cpu != TLS->rseq::cpu_id)
>  *                             goto abort_ip;
>  *   [3]               <last_instruction_in_cs>
>  *   [post_commit_ip]  ----------------------------
>  *   [4]               TLS->rseq::rseq_cs = NULL
> 
> But as a fast-path optimization, [4] is not entirely needed because the rseq_cs
> descriptor contains information about the instruction pointer range of the critical
> section. Therefore, userspace can omit [4], but if the kernel never clears it, it
> means that it will have to re-read the rseq_cs descriptor's content each time it
> needs to check it to confirm that it is not nested over a rseq c.s..
> 
> So making the kernel lazily clear the rseq_cs pointer is just an optimization which
> ensures that the kernel won't do useless work the next time it needs to check
> rseq_cs, given that it has already validated that the userspace code is currently
> not within the rseq c.s. currently advertised by the rseq_cs field.

Thanks for the explanation, much appreciated!