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

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Message-ID: <CABgObfZFgHY_ybfSnyzEF1dFr5c1s=_r+tAnHa6Q7rzFUUdt3g@mail.gmail.com>
Date: Mon, 6 Oct 2025 13:19:39 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Naman Jain <namjain@...ux.microsoft.com>, Sean Christopherson <seanjc@...gle.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 Mon, Oct 6, 2025 at 1:10 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Oct 06, 2025 at 04:20:03PM +0530, Naman Jain wrote:
>
> > I am facing issues in this approach, after moving the assembly code to a
> > separate file, using static calls, and making it noinstr.
> >
> > We need to make a call to STATIC_CALL_TRAMP_STR(hv_hypercall_pg + offset) in
> > the assembly code. This offset is populated at run time in the driver, so I
> > have to pass this offset to the assembly function via function parameters or
> > a shared variable. This leaves noinstr section and results in below warning:
> >
> > [1]: vmlinux.o: warning: objtool: __mshv_vtl_return_call+0x4f: call to
> > mshv_vtl_call_addr() leaves .noinstr.text section
> >
> >
> > To fix this, one of the ways was to avoid making indirect calls. So I used
> > EXPORT_STATIC_CALL to export the static call *trampoline and key* for the
> > static call we created in C driver. Then I figured, we could simply call
> > __SCT__<static_callname> in assembly code and it should work fine. But then
> > it leads to this error in objtool.
>
> Easiest solution is to create a second static_call and have
> hv_set_hypercall_pg() set that to +offset.

Yes, my idea was to add +offset directly in the static_call_update, as
you sketched below. Sorry if that wasn't too clear. I didn't think of
using a static call also for the base of the page, since that's not
what the assembly code needs.

And I think we agree that you absolutely don't want indirect calls, as
that makes register usage much simpler. This way static calls end up
killing multiple birds with a stone.

> Also, what's actually in that hypercall page that is so magical and
> can't just be an ALTERNATIVE() ?

Do you mean an alternative for VMCALL vs. VMMCALL? If so, that's just
not guaranteed to work: "An attempt to invoke a hypercall by any other
means (for example, copying the code from the hypercall code page to
an alternate location and executing it from there) might result in an
undefined operation (#UD) exception" (or it might not).

Paolo