linux-hyperv.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Michael Kelley <mikelley@microsoft.com>
To: Praveen Kumar <kumarpraveen@linux.microsoft.com>,
	Wei Liu <wei.liu@kernel.org>
Cc: "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	KY Srinivasan <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	Dexuan Cui <decui@microsoft.com>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"mingo@redhat.com" <mingo@redhat.com>,
	"bp@alien8.de" <bp@alien8.de>, "x86@kernel.org" <x86@kernel.org>,
	"hpa@zytor.com" <hpa@zytor.com>,
	"viremana@linux.microsoft.com" <viremana@linux.microsoft.com>,
	Sunil Muthuswamy <sunilmut@microsoft.com>,
	"nunodasneves@linux.microsoft.com"
	<nunodasneves@linux.microsoft.com>
Subject: RE: [PATCH] hyperv: root partition faults writing to VP ASSIST MSR PAGE
Date: Wed, 21 Jul 2021 15:07:22 +0000	[thread overview]
Message-ID: <MWHPR21MB1593448243F5DC37B0DC15D0D7E39@MWHPR21MB1593.namprd21.prod.outlook.com> (raw)
In-Reply-To: <497414c3-3782-26ad-3b41-105ee12098d4@linux.microsoft.com>

From: Praveen Kumar <kumarpraveen@linux.microsoft.com> Sent: Wednesday, July 21, 2021 4:32 AM
> 
> On 21-07-2021 15:40, Wei Liu wrote:
> > On Wed, Jul 21, 2021 at 12:42:52PM +0530, Praveen Kumar wrote:
> >> On 21-07-2021 09:40, Michael Kelley wrote:
> >>> From: Wei Liu <wei.liu@kernel.org> Sent: Tuesday, July 20, 2021 9:29 AM
> >>>>
> >>>> On Tue, Jul 20, 2021 at 04:20:44PM +0000, Michael Kelley wrote:
> >>>>> From: Wei Liu <wei.liu@kernel.org> Sent: Tuesday, July 20, 2021 6:35 AM
> >>>>>>
> >>>>>> On Tue, Jul 20, 2021 at 06:55:56PM +0530, Praveen Kumar wrote:
> >>>>>> [...]
> >>>>>>>>
> >>>>>>>>> +	if (hv_root_partition &&
> >>>>>>>>> +	    ms_hyperv.features & HV_MSR_APIC_ACCESS_AVAILABLE) {
> >>>>>>>>
> >>>>>>>> Is HV_MSR_APIC_ACCESS_AVAILABLE a root only flag? Shouldn't non-root
> >>>>>>>> kernel check this too?
> >>>>>>>
> >>>>>>> Yes, you are right. Will update this in v2. thanks.
> >>>>>>
> >>>>>> Please split adding this check to its own patch.
> >>>>>>
> >>>>>> Ideally one patch only does one thing.
> >>>>>>
> >>>>>> Wei.
> >>>>>>
> >>>>>
> >>>>> I was just looking around in the Hyper-V TLFS, and I didn't see
> >>>>> anywhere that the ability to set up a VP Assist page is dependent
> >>>>> on HV_MSR_APIC_ACCESS_AVAILABLE.  Or did I just miss it?
> >>>>
> >>>> The feature bit Praveen used is wrong and should be fixed.
> >>>>
> >>>> Per internal discussion this is gated by the AccessIntrCtrlRegs bit.
> >>>>
> >>>> Wei.
> >>>>
> >>>
> >>> The AccessIntrCtrlRegs bit *is* HV_MSR_APIC_ACCESS_AVAILABLE.
> >>> Both are defined as bit 4 of the Partition Privilege flags.  :-)   I don't
> >>> know why the names don't line up.   Even so, it's not clear to me that
> >>> AccessIntrCtrlRegs has any bearing on the VP Assist page.  I see this
> >>> description of AccessIntrCtrlRegs:
> >>>
> >>
> >> Yup, what I understood as well, this is the one required one for Partition Privilege Flags (4th bit), however, cannot
> comment on the naming convention.
> >>
> >>      5 /* Virtual APIC assist and VP assist page registers available */
> >>      4 #define HV_MSR_APIC_ACCESS_AVAILABLE            BIT(4)
> >>
> >
> > Urgh, okay. It is my fault for not reading the code closely. Sorry for
> > the confusion.
> >
> >>> The partition has access to the synthetic MSRs associated with the
> >>> APIC (HV_X64_MSR_EOI, HV_X64_MSR_ICR and HV_X64_MSR_TPR).
> >>> If this flag is cleared, accesses to these MSRs results in a #GP fault if
> >>> the MSR intercept is not installed.
> >>>
> >>
> >> As per what I also understood from the TLFS doc,that we let partition
> >> access the MSR and do a fault.  However, the point is, does it make
> >> sense to allocate page for vp assist and perform action which is meant
> >> to fail when the flag is cleared ?
> >
> > Like Michael said, there are some other things that are not tied to that
> > particular bit. We should get more clarity on what gates what.  Perhaps
> > that privilege bit only controls access to the EOI assist bit and the
> > other things in the VP assist page are gated by other privilege bits.
> > This basically means we should setup the page when there is at least one
> > thing in that page can be used.
> >
> > This is mostly an orthogonal issue from the one we want to fix. In
> > the interest of making progress we can drop the new check for now and
> > just add a root specific path for setting up and tearing down the VP
> > assist pages.
> >
> > How does that sound?
> >
> 
> Sounds good to me. Thanks Wei.
> 

Work for me as well.  Praveen -- The inconsistency in the name is
historical, and not something that needs to be changed now.  My
comment was just musing, not something actionable. :-)

Michael

      reply	other threads:[~2021-07-21 15:07 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-19 18:51 Praveen Kumar
2021-07-20 11:20 ` Wei Liu
2021-07-20 13:25   ` Praveen Kumar
2021-07-20 13:35     ` Wei Liu
2021-07-20 16:20       ` Michael Kelley
2021-07-20 16:29         ` Wei Liu
2021-07-21  4:10           ` Michael Kelley
2021-07-21  7:12             ` Praveen Kumar
2021-07-21 10:10               ` Wei Liu
2021-07-21 11:32                 ` Praveen Kumar
2021-07-21 15:07                   ` Michael Kelley [this message]

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=MWHPR21MB1593448243F5DC37B0DC15D0D7E39@MWHPR21MB1593.namprd21.prod.outlook.com \
    --to=mikelley@microsoft.com \
    --cc=bp@alien8.de \
    --cc=decui@microsoft.com \
    --cc=haiyangz@microsoft.com \
    --cc=hpa@zytor.com \
    --cc=kumarpraveen@linux.microsoft.com \
    --cc=kys@microsoft.com \
    --cc=linux-hyperv@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mingo@redhat.com \
    --cc=nunodasneves@linux.microsoft.com \
    --cc=sthemmin@microsoft.com \
    --cc=sunilmut@microsoft.com \
    --cc=tglx@linutronix.de \
    --cc=viremana@linux.microsoft.com \
    --cc=wei.liu@kernel.org \
    --cc=x86@kernel.org \
    --subject='RE: [PATCH] hyperv: root partition faults writing to VP ASSIST MSR PAGE' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).