linux-kernel - Re: [PATCH] x86/hyperv: Export hv_hypercall

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Message-ID: <aMl5ulY1K7cKcMfo@google.com>
Date: Tue, 16 Sep 2025 07:52:42 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: Roman Kisel <romank@...ux.microsoft.com>, Peter Zijlstra <peterz@...radead.org>, 
	Naman Jain <namjain@...ux.microsoft.com>, "K . Y . Srinivasan" <kys@...rosoft.com>, 
	Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>, 
	Dexuan Cui <decui@...rosoft.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H . Peter Anvin" <hpa@...or.com>, linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org, 
	mhklinux@...look.com
Subject: Re: [PATCH] x86/hyperv: Export hv_hypercall_pg unconditionally

On Tue, Sep 16, 2025, Paolo Bonzini wrote:
> On 8/27/25 01:04, Roman Kisel wrote:
> > On 8/26/2025 5:07 AM, Peter Zijlstra wrote:
> > > I do not know what OpenHCL is. Nor is it clear from the code what NMIs
> > > can't happen. Anyway, same can be achieved with breakpoints / kprobes.
> > > You can get a trap after setting CR2 and scribble it.
> > > 
> > > You simply cannot use CR2 this way.
> > 
> > The code in question runs with interrupts disabled, and the kernel runs
> > without the memory swapping when using that module - the kernel is
> > a firmware to host a vTPM for virtual machines. Somewhat similar to SMM.
> > That should've been reflected somewhere in the comments and in Kconfig,
> > we could do better. All in all, the page fault cannot happen in that
> > path thus CR2 won't be trashed.
> > 
> > Nor this kind of code can be stepped through in a self-hosted
> > kernel debugger like kgdb. There are other examples of such code iiuc:
> 
> As Sean mentioned, you do have to make sure that this is annotated as
> noinstr (not instrumentable).  And also just use assembly - KVM started with
> a similar asm block, though without the sketchy "register asm",

Ooh, yeah, don't use "register asm".  I missed that when I peeked at the code.
Using "register asm" will most definitely cause problems, because the compiler
doesn't track usage in C code, i.e. will happily use the GPR and clobber your
asm value in the process.  That inevitably leads to very confusing and somewhat
transient errors.  E.g. if someone inserts a printk for debugging, the call to
printk can clobber the very state it's trying to print.

> and I was initially skeptical but using a dedicated .S file was absolutely
> the right thing to do.

+1000 to putting the assembly in a .S file.  I too was a bit skeptical about
moving the entire sequence into proper assembly; thankfully, some non-KVM folks
talked us into it :-)