linux-kernel - Re: [PATCH] Updates to Xen hypercall preemption

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Message-ID: <20230622131445.GQ4253@hirez.programming.kicks-ass.net>
Date:   Thu, 22 Jun 2023 15:14:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     Juergen Gross <jgross@...e.com>, Per Bilse <Per.Bilse@...rix.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        "open list:X86 ENTRY CODE" <linux-kernel@...r.kernel.org>,
        "moderated list:XEN HYPERVISOR INTERFACE" 
        <xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH] Updates to Xen hypercall preemption

On Thu, Jun 22, 2023 at 02:05:13PM +0100, Andrew Cooper wrote:
> On 22/06/2023 9:26 am, Peter Zijlstra wrote:
> > On Thu, Jun 22, 2023 at 07:22:53AM +0200, Juergen Gross wrote:
> >
> >> The hypercalls we are talking of are synchronous ones. They are running
> >> in the context of the vcpu doing the call (like a syscall from userland is
> >> running in the process context).
> > (so time actually passes from the guest's pov?)
> 
> Yes.  And in principle it's wired into stolen time.

Hmm, that means that if the scheduler accounts for stolen time (see
update_rq_clock_task(), PARAVIRT_TIME_ACCOUNTING hunk) then it appears
as if no time has passed, which might affect preemption decisions.

Oh well, we'll think about it when it's shown to be a problem I suppose
:-)