Re: [PATCH v2 for-4.15] x86/msr: introduce an option for HVM relaxed rdmsr behavior

From: "Roger Pau Monné" <roger.pau@citrix.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	<xen-devel@lists.xenproject.org>,
	Ian Jackson <iwj@xenproject.org>, Wei Liu <wl@xen.org>,
	Anthony PERARD <anthony.perard@citrix.com>,
	Jan Beulich <jbeulich@suse.com>,
	Jun Nakajima <jun.nakajima@intel.com>,
	Kevin Tian <kevin.tian@intel.com>
Subject: Re: [PATCH v2 for-4.15] x86/msr: introduce an option for HVM relaxed rdmsr behavior
Date: Fri, 5 Mar 2021 09:26:36 +0100	[thread overview]
Message-ID: <YEHrPIL1EhWjWY+9@Air-de-Roger> (raw)
In-Reply-To: <c8bcb182-2f6c-accf-6b94-440de8af3617@citrix.com>

On Thu, Mar 04, 2021 at 11:28:16PM +0000, Andrew Cooper wrote:
> On 04/03/2021 23:09, Boris Ostrovsky wrote:
> > On 3/4/21 9:47 AM, Roger Pau Monne wrote:
> >> Introduce an option to allow selecting a less strict behaviour for
> >> rdmsr accesses targeting a MSR not explicitly handled by Xen. Since
> >> commit 84e848fd7a162f669 accesses to MSRs not explicitly handled by
> >> Xen result in the injection of a #GP to the guest. This is a behavior
> >> change since previously a #GP was only injected if accessing the MSR
> >> on the real hardware will also trigger a #GP.
> >>
> >> This commit attempts to offer a fallback option similar to the
> >> previous behavior. Note however that the value of the underlying MSR
> >> is never leaked to the guest, as the newly introduced option only
> >> changes whether a #GP is injected or not.
> >>
> >> Long term the plan is to properly handle all the MSRs, so the option
> >> introduced here should be considered a temporary resort for OSes that
> >> don't work properly with the new MSR policy. Any OS that requires this
> >> option to be enabled should be reported to
> >> xen-devel@lists.xenproject.org.
> >>
> >> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> >> ---
> >> Changes since v1:
> >>  - Only apply the option to HVM guests.
> >>  - Only apply the special handling to MSR reads.
> >>  - Sanitize the newly introduced flags field.
> >>  - Print a warning message when the option is used.
> >> ---
> >> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> >> ---
> >> Boris, could you please test with Solaris to see if this fixes the
> >> issue?
> >
> > Yes, still works. (It worked especially well after I noticed new option name ;-))
> 
> I'm afraid I want to break and rework how this bugfix happens.  Solaris
> is still broken on all older branches and this isn't a suitable fix to
> backport.

Right, I think that's the reactive part of the fixing that we spoke
with Jan, but I think we would still need something similar to
rdmsr_relaxed.

There's at least one other MSR which Jan identified that we also want
to handle MSR_K7_HWCR.

Thanks, Roger.