From:   "Li, Xin3" <xin3.li@intel.com>
To:     Peter Zijlstra <peterz@infradead.org>
CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>,
        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
        "hpa@zytor.com" <hpa@zytor.com>,
        "andrew.cooper3@citrix.com" <andrew.cooper3@citrix.com>,
        "Christopherson,, Sean" <seanjc@google.com>,
        "pbonzini@redhat.com" <pbonzini@redhat.com>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>
Subject: RE: [PATCH v5 20/34] x86/fred: add a machine check entry stub for
 FRED
Thread-Topic: [PATCH v5 20/34] x86/fred: add a machine check entry stub for
 FRED
Thread-Index: AQHZUKIOC/3kvxGGb0mH1uURdpTy+K8D6OGAgACEs5CAAJgogIAAf+hg
Date:   Tue, 21 Mar 2023 16:38:46 +0000
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> On Tue, Mar 21, 2023 at 12:04:47AM +0000, Li, Xin3 wrote:
> > > > Unlike IDT, no need to save/restore dr7 in FRED machine check handl=
er.
> > >
> > > Given how fragile MCE is, the question should be, do we ever want hw
> > > breakpoints to happen while it is running?
> >
> > HW breakpoints still work if they are properly configured.
> >
> > > If the hw-breakpoint handler trips on the same memory fail that got
> > > us into the mce the first time, we're dead.
> >
> > Right.
> >
> > Unless the MCIP bit is turned off any subsequent #MC goes to shutdown
> > ("machine is screwed").
> >
> > It's the kernel debugger's responsibility to decide how to proceed in
> > such cases. But if the kernel debugger itself is in a screwed memory
> > region, we are soooooo dead.
>=20
> Yeah, so I would much prefer, for robustness sake, to start out with not =
allowing
> #DB in MCE -- much like today.

Will disable #DB inside #MCE then.