Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   KY Srinivasan <kys@microsoft.com>
To:     "Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        "x86@kernel.org" <x86@kernel.org>
CC:     "devel@linuxdriverproject.org" <devel@linuxdriverproject.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Tianyu Lan <Tianyu.Lan@microsoft.com>
Subject: RE: [PATCH] x86/hyper-v: use cheaper
 HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST,SPACE} hypercalls when possible
Thread-Topic: [PATCH] x86/hyper-v: use cheaper
 HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST,SPACE} hypercalls when possible
Thread-Index: AQHUBMYlSJQ3WMa140uivtTO6PwhI6Rn5ECAgAAF1FA=
Date:   Tue, 19 Jun 2018 18:20:36 +0000
Message-ID: <MW2PR2101MB111340FC6B6AFA7CEC494498A0700@MW2PR2101MB1113.namprd21.prod.outlook.com>
References: <20180615163010.20381-1-vkuznets@redhat.com>
 <SN6PR2101MB11209236A3C18C7CA01F4061DC700@SN6PR2101MB1120.namprd21.prod.outlook.com>
In-Reply-To: <SN6PR2101MB11209236A3C18C7CA01F4061DC700@SN6PR2101MB1120.namprd21.prod.outlook.com>
Accept-Language: en-US
Content-Language: en-US
received-spf: None (protection.outlook.com: microsoft.com does not designate
 permitted sender hosts)
spamdiagnosticoutput: 1:99
spamdiagnosticmetadata: NSPM
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
X-MS-Exchange-CrossTenant-Network-Message-Id: 90281867-d94f-4a62-2417-08d5d6115ae0
X-MS-Exchange-CrossTenant-originalarrivaltime: 19 Jun 2018 18:20:36.8679
 (UTC)
X-MS-Exchange-CrossTenant-fromentityheader: Hosted
X-MS-Exchange-CrossTenant-id: 72f988bf-86f1-41af-91ab-2d7cd011db47
X-MS-Exchange-Transport-CrossTenantHeadersStamped: MW2PR2101MB0908
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org


> -----Original Message-----
> From: Michael Kelley (EOSG)
> Sent: Tuesday, June 19, 2018 10:57 AM
> To: Vitaly Kuznetsov <vkuznets@redhat.com>; x86@kernel.org
> Cc: devel@linuxdriverproject.org; linux-kernel@vger.kernel.org; KY
> Srinivasan <kys@microsoft.com>; Haiyang Zhang
> <haiyangz@microsoft.com>; Stephen Hemminger
> <sthemmin@microsoft.com>; Thomas Gleixner <tglx@linutronix.de>; Ingo
> Molnar <mingo@redhat.com>; H. Peter Anvin <hpa@zytor.com>; Tianyu Lan
> <Tianyu.Lan@microsoft.com>
> Subject: RE: [PATCH] x86/hyper-v: use cheaper
> HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST,SPACE} hypercalls when possible
>=20
> > -----Original Message-----
> > From: linux-kernel-owner@vger.kernel.org <linux-kernel-
> owner@vger.kernel.org> On Behalf
> > Of Vitaly Kuznetsov
> > Sent: Friday, June 15, 2018 9:30 AM
> > To: x86@kernel.org
> > Cc: devel@linuxdriverproject.org; linux-kernel@vger.kernel.org; KY
> Srinivasan
> > <kys@microsoft.com>; Haiyang Zhang <haiyangz@microsoft.com>;
> Stephen Hemminger
> > <sthemmin@microsoft.com>; Thomas Gleixner <tglx@linutronix.de>; Ingo
> Molnar
> > <mingo@redhat.com>; H. Peter Anvin <hpa@zytor.com>; Tianyu Lan
> > <Tianyu.Lan@microsoft.com>
> > Subject: [PATCH] x86/hyper-v: use cheaper
> HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST,SPACE}
> > hypercalls when possible
> >
> > While working on Hyper-V style PV TLB flush support in KVM I noticed th=
at
> > real Windows guests use TLB flush hypercall in a somewhat smarter way:
> when
> > the flush needs to be performed on a subset of first 64 vCPUs or on all
> > present vCPUs Windows avoids more expensive hypercalls which support
> > sparse CPU sets and uses their 'cheap' counterparts. This means that
> > HV_X64_EX_PROCESSOR_MASKS_RECOMMENDED name is actually a
> misnomer: EX
> > hypercalls (which support sparse CPU sets) are "available", not
> > "recommended". This makes sense as they are actually harder to parse.
> >
> > Nothing stops us from being equally 'smart' in Linux too. Switch to
> > doing cheaper hypercalls whenever possible.
> >
> > Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> > ---
>=20
> This is a good idea.  We should probably do the same with the hypercalls =
for
> sending
> IPIs -- try the simpler version first and move to the more complex _EX
> version only
> if necessary.
I am not sure if this would work correctly. When I was developing the IPI e=
nlightenment,=20
what I remember was that the guest is expected to use the API recommended b=
y the Hypervisor.

K. Y=20
>=20
> A complication:  We've recently found a problem with the code for doing I=
PI
> hypercalls, and the bug affects the TLB flush code as well.  As secondary=
 CPUs
> are started, there's a window of time where the hv_vp_index entry for a
> secondary CPU is uninitialized.  We are seeing IPIs happening in that win=
dow,
> and
> the IPI hypercall code uses the uninitialized hv_vp_index entry.   Same t=
hing
> could
> happen with the TLB flush hypercall code.  I didn't actually see any
> occurrences of
> the TLB case in my tracing, but we should fix it anyway in case a TLB flu=
sh gets
> added at some point in the future.
>=20
> KY has a patch coming.  In the patch, hv_cpu_number_to_vp_number()
> and cpumask_to_vpset() can both return U32_MAX if they encounter an
> uninitialized hv_vp_index entry, and the code needs to be able to bail ou=
t to
> the native functions for that particular IPI or TLB flush operation.  Onc=
e the
> initialization of secondary CPUs is complete, the uninitialized situation=
 won't
> happen again, and the hypercall path will always be used.
>=20
> We'll need to coordinate on these patches.  Be aware that the IPI flavor =
of
> the
> bug is currently causing random failures when booting 4.18 RC1 on Hyper-V
> VMs
> with large vCPU counts.
>=20
> Reviewed-by:  Michael Kelley <mikelley@microsoft.com>