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

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Message-ID: <20250918064713.GW3419281@noisy.programming.kicks-ass.net>
Date: Thu, 18 Sep 2025 08:47:13 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Naman Jain <namjain@...ux.microsoft.com>
Cc: Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Roman Kisel <romank@...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 Thu, Sep 18, 2025 at 11:33:18AM +0530, Naman Jain wrote:

> Thank you so much Sean and Paolo for your valuable inputs. I will try
> out these things. Summarizing the suggestions here:
> * Use noinstr (no instrumentation)
> * Have separate .S file
> * Don't use "register asm".
> * Use static calls for solving IBT problems
> * RAX:RCX is probably ok to be used, considering ABI. Whether we would still
> need to use STACK_FRAME_NON_STANDARD, I am not sure, but I will see based on
> how it goes.
> 
> I hope this addresses the concerns Peter raised. If there's anything I might
> have missed, I'm happy to make further adjustments if needed.

It would be a definite improvement. I'm just *really* sad people still
create interfaces like this, even though we've known for years how bad
they are.

At some point we've really have to push back and say enough is enough.