Re: [PATCH 5/6] KVM: x86/VMX: add kvm_vmx_reinject_nmi_irq() for NMI/IRQ reinjection

From: Peter Zijlstra <peterz@infradead.org>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Sean Christopherson <seanjc@google.com>,
	"Li, Xin3" <xin3.li@intel.com>,
	"linux-kernel@vger.kernnel.org" <linux-kernel@vger.kernnel.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>,
	"pbonzini@redhat.com" <pbonzini@redhat.com>,
	"Tian, Kevin" <kevin.tian@intel.com>
Subject: Re: [PATCH 5/6] KVM: x86/VMX: add kvm_vmx_reinject_nmi_irq() for NMI/IRQ reinjection
Date: Fri, 11 Nov 2022 11:45:53 +0100	[thread overview]
Message-ID: <Y24n4bHoFBuHVid5@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <3A1B7743-9448-405A-8BE4-E1BDAB4D62F8@zytor.com>

On Fri, Nov 11, 2022 at 01:29:35AM -0800, H. Peter Anvin wrote:
> On November 11, 2022 1:19:23 AM PST, Peter Zijlstra <peterz@infradead.org> wrote:
> >On Thu, Nov 10, 2022 at 08:53:09PM +0000, Sean Christopherson wrote:
> >> On Thu, Nov 10, 2022, Li, Xin3 wrote:
> >
> >> > > > + * call thus the values in the pt_regs structure are not used in
> >> > > > + * executing NMI/IRQ handlers,
> >> > > 
> >> > > Won't this break stack traces to some extent?
> >> > > 
> >> > 
> >> > The pt_regs structure, and its IP/CS, is NOT part of the call stack, thus
> >> > I don't see a problem. No?
> >
> >I'm not sure what Xin3 is trying to say, but NMI/IRQ handers use pt_regs
> >a *LOT*. pt_regs *MUST* be correct.
> 
> What is "correct" in this context? 

I don't know since I don't really speak virt, but I could image the
regset that would match the vmrun (or whatever intel decided to call
that again) instruction.

> Could you describe what aspects of
> the register image you rely on, and what you expect them to be?

We rely on CS,IP,FLAGS,SS,SP to be coherent and usable at the very least
(must be able to start an unwind from it). But things like perf (NMI)
access *all* of them and possibly copy them out to userspace. Perf can
also try and use the segment registers in order to try and establish a
linear address.

Some exceptions (#GP) access whatever is needed to fully decode and
emulate the instruction (IOPL,UMIP,etc..) including the segment
registers.

> Currently KVM basically stuff random data into pt_regs; this at least
> makes it explicitly zero.

:-( Both is broken. Once again proving to me that virt is a bunch of
duck-tape at best.