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

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Date:   Thu, 22 Jun 2023 10:26:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juergen Gross <jgross@...e.com>
Cc:     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 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?)

> The hypervisor will return to guest context from time to time by modifying
> the registers such that the guest will do the hypercall again with different
> input values for the hypervisor, resulting in a proper continuation of the
> hypercall processing.

Eeeuw.. that's pretty terrible. And changing this isn't in the cards,
like at all?

That is, why isn't this whole thing written like:

	for (;;) {
		ret = hypercall(foo);
		if (ret == -EAGAIN) {
			cond_resched();
			continue;
		}
		break;
	}

> It is an awful interface and I agree that switching to full preemption in
> dom0 seems to be the route which we should try to take.

Well, I would very strongly suggest the route to take is to scrap the
whole thing and invest in doing something saner so we don't have to jump
through hoops like this.

This is quite possibly the worst possible interface for this Xen could
have come up with -- awards material for sure.

> The downside would be that some workloads might see worse performance
> due to backend I/O handling might get preempted.

Is that an actual concern? Mark this a legaxy inteface and anybody who
wants to get away from it updates.

> Just thinking - can full preemption be enabled per process?

Nope, that's a system wide thing. Preemption is something that's driven
by the requirements of the tasks that preempt, not something by the
tasks that get preempted.

Andy's idea of having that thing intercepted as an exception (EXTABLE
like) and relocating the IP to a place that does cond_resched() before
going back is an option.. gross, but possibly better, dunno.

Quite the mess indeed :/