From: Greg Kurz <groug@kaod.org>
To: "Cédric Le Goater" <clg@kaod.org>
Cc: Gustavo Romero <gromero@linux.ibm.com>,
qemu-ppc@nongnu.org, qemu-devel@nongnu.org,
David Gibson <david@gibson.dropbear.id.au>
Subject: Re: [PATCH v2 3/6] spapr/xive: Add a warning when StoreEOI is activated on POWER9 CPUs
Date: Wed, 7 Oct 2020 10:56:30 +0200 [thread overview]
Message-ID: <20201007105630.64e2f0c3@bahia.lan> (raw)
In-Reply-To: <e8013373-4536-08e4-27c9-8c8b8f46f768@kaod.org>
On Tue, 6 Oct 2020 19:03:28 +0200
Cédric Le Goater <clg@kaod.org> wrote:
> On 10/6/20 6:58 PM, Greg Kurz wrote:
> > On Mon, 5 Oct 2020 18:51:44 +0200
> > Cédric Le Goater <clg@kaod.org> wrote:
> >
> >> StoreEOI on POWER9 CPUs is racy because load-after-store ordering is
> >> not enforced.
> >>
> >> Signed-off-by: Cédric Le Goater <clg@kaod.org>
> >> ---
> >> hw/ppc/spapr_caps.c | 9 +++++++++
> >> 1 file changed, 9 insertions(+)
> >>
> >> diff --git a/hw/ppc/spapr_caps.c b/hw/ppc/spapr_caps.c
> >> index b0a9d0227db2..9251badbdc27 100644
> >> --- a/hw/ppc/spapr_caps.c
> >> +++ b/hw/ppc/spapr_caps.c
> >> @@ -549,6 +549,15 @@ static void cap_storeeoi_apply(SpaprMachineState *spapr, uint8_t val,
> >> error_setg(errp, "StoreEOI not supported by KVM");
> >> return;
> >> }
> >> +
> >> + /*
> >> + * load-after-store ordering is not enforced on POWER9 CPUs
> >> + * and StoreEOI can be racy.
> >> + */
> >> + if (!ppc_type_check_compat(machine->cpu_type, CPU_POWERPC_LOGICAL_3_10,
> >> + 0, spapr->max_compat_pvr)) {
> >> + warn_report("StoreEOI on a POWER9 CPU is unsafe on KVM.");
> >
The error message should mention XIVE KVM device actually since this only
depends on kernel-irqchip.
> > It all boils down to what "unsafe" really means here... if the outcome is
> > "very likely hang the guest" as soon as it starts doing I/O, shouldn't
> > we error out instead ? What is the motivation to use StoreEOI if the
> > processor doesn't really support it ?
>
> We use it in the lab on P9. We have never seen it failed even under stress.
> But there is a possible race in the logic.
>
> C.
Thinking again. P9 boston systems on the field only use emulated XIVE and
we certainly want to be able to migrate to P9 systems that use in-kernel
XIVE and vice-versa. So, even if StoreEOI is technically supported by
the emulated XIVE, maybe we should disallow it anyway ?
next prev parent reply other threads:[~2020-10-07 8:58 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-05 16:51 [PATCH v2 0/6] spapr/xive: Activate StoreEOI in P10 compat guests Cédric Le Goater
2020-10-05 16:51 ` [PATCH v2 1/6] spapr/xive: Introduce a StoreEOI capability Cédric Le Goater
2020-10-06 16:42 ` Greg Kurz
2020-10-07 5:59 ` Cédric Le Goater
2020-10-07 7:24 ` Greg Kurz
2020-10-05 16:51 ` [PATCH v2 2/6] spapr/xive: Add a warning when StoreEOI is activated on POWER8 CPUs Cédric Le Goater
2020-10-06 16:52 ` Greg Kurz
2020-10-05 16:51 ` [PATCH v2 3/6] spapr/xive: Add a warning when StoreEOI is activated on POWER9 CPUs Cédric Le Goater
2020-10-06 16:58 ` Greg Kurz
2020-10-06 17:03 ` Cédric Le Goater
2020-10-07 8:56 ` Greg Kurz [this message]
2020-10-07 9:21 ` Cédric Le Goater
2020-10-05 16:51 ` [PATCH v2 4/6] spapr/xive: Enforce load-after-store ordering Cédric Le Goater
2020-10-06 17:02 ` Greg Kurz
2020-10-05 16:51 ` [PATCH v2 5/6] spapr/xive: Activate StoreEOI at the source level Cédric Le Goater
2020-10-06 17:06 ` Greg Kurz
2020-10-06 17:41 ` Cédric Le Goater
2020-10-07 7:26 ` Greg Kurz
2020-10-05 16:51 ` [PATCH v2 6/6] spapr/xive: Introduce a new CAS value for the StoreEOI capability Cédric Le Goater
2020-10-06 17:39 ` Greg Kurz
2020-10-06 17:56 ` Cédric Le Goater
2020-10-07 13:43 ` Greg Kurz
2020-10-07 14:28 ` Cédric Le Goater
2020-10-09 0:23 ` [PATCH v2 0/6] spapr/xive: Activate StoreEOI in P10 compat guests David Gibson
2020-10-09 5:57 ` Cédric Le Goater
2020-10-12 5:38 ` David Gibson
2020-11-02 13:22 ` Cédric Le Goater
2020-11-23 6:44 ` David Gibson
2020-11-23 11:16 ` Cédric Le Goater
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