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

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Message-ID: <20250825094247.GU3245006@noisy.programming.kicks-ass.net>
Date: Mon, 25 Aug 2025 11:42:47 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Naman Jain <namjain@...ux.microsoft.com>
Cc: "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 Mon, Aug 25, 2025 at 11:22:08AM +0530, Naman Jain wrote:
> With commit 0e20f1f4c2cb ("x86/hyperv: Clean up hv_do_hypercall()"),
> config checks were added to conditionally restrict export
> of hv_hypercall_pg symbol at the same time when a usage of that symbol
> was added in mshv_vtl_main.c driver. This results in missing symbol
> warning when mshv_vtl_main is compiled. Change the logic to
> export it unconditionally.
> 
> Fixes: 96a1d2495c2f ("Drivers: hv: Introduce mshv_vtl driver")
> Signed-off-by: Naman Jain <namjain@...ux.microsoft.com>

Oh gawd, that commit is terrible and adds yet another hypercall
interface.

I would argue the proper fix is moving the whole of mshv_vtl_return()
into the kernel proper and doing it like hv_std_hypercall() on x86_64.

Additionally, how is that function not utterly broken? What happens if
an interrupt or NMI comes in after native_write_cr2() and before the
actual hypercall does VMEXIT and trips a #PF?

And an rax:rcx return, I though the canonical pair was AX,DX !?!?

Also, that STACK_FRAME_NON_STANDARD() annotation is broken, this must
not be used for anything that can end up in vmlinux.o -- that is, the
moment you built-in this driver (=y) this comes unstuck.

The reason you're getting warnings is because you're violating the
normal calling convention and scribbling BP, we yelled at the TDX guys
for doing this, now you're getting yelled at. WTF !?!

Please explain how just shutting up objtool makes the unwind work when
the NMI hits your BP scribble?

All in all, I would suggest fixing this by reverting that patch and
trying again after fixing the calling convention of that hypercall.

Yours grumpy..