linux-kernel - Re: [PATCH 4/6] x86,hyperv: Clean up hv_do

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Message-ID: <20250414140812.GH5600@noisy.programming.kicks-ass.net>
Date: Mon, 14 Apr 2025 16:08:12 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Uros Bizjak <ubizjak@...il.com>
Cc: x86@...nel.org, kys@...rosoft.com, haiyangz@...rosoft.com,
	wei.liu@...nel.org, decui@...rosoft.com, tglx@...utronix.de,
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
	hpa@...or.com, jpoimboe@...nel.org,
	pawan.kumar.gupta@...ux.intel.com, seanjc@...gle.com,
	pbonzini@...hat.com, ardb@...nel.org, kees@...nel.org,
	Arnd Bergmann <arnd@...db.de>, gregkh@...uxfoundation.org,
	linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, linux-efi@...r.kernel.org,
	samitolvanen@...gle.com, ojeda@...nel.org
Subject: Re: [PATCH 4/6] x86,hyperv: Clean up hv_do_hypercall()

On Mon, Apr 14, 2025 at 04:06:41PM +0200, Uros Bizjak wrote:
> 
> 
> On 14. 04. 25 13:11, Peter Zijlstra wrote:
> > What used to be a simple few instructions has turned into a giant mess
> > (for x86_64). Not only does it use static_branch wrong, it mixes it
> > with dynamic branches for no apparent reason.
> > 
> > Notably it uses static_branch through an out-of-line function call,
> > which completely defeats the purpose, since instead of a simple
> > JMP/NOP site, you get a CALL+RET+TEST+Jcc sequence in return, which is
> > absolutely idiotic.
> > 
> > Add to that a dynamic test of hyperv_paravisor_present, something
> > which is set once and never changed.
> > 
> > Replace all this idiocy with a single direct function call to the
> > right hypercall variant.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >   arch/x86/hyperv/hv_init.c       |   21 ++++++
> >   arch/x86/hyperv/ivm.c           |   14 ++++
> >   arch/x86/include/asm/mshyperv.h |  137 +++++++++++-----------------------------
> >   arch/x86/kernel/cpu/mshyperv.c  |   18 +++--
> >   4 files changed, 88 insertions(+), 102 deletions(-)
> > 
> > --- a/arch/x86/hyperv/hv_init.c
> > +++ b/arch/x86/hyperv/hv_init.c
> > @@ -35,7 +35,28 @@
> >   #include <linux/highmem.h>
> >   void *hv_hypercall_pg;
> > +
> > +#ifdef CONFIG_X86_64
> > +u64 hv_pg_hypercall(u64 control, u64 param1, u64 param2)
> > +{
> > +	u64 hv_status;
> > +
> > +	if (!hv_hypercall_pg)
> > +		return U64_MAX;
> > +
> > +	register u64 __r8 asm("r8") = param2;
> > +	asm volatile (CALL_NOSPEC
> > +		      : "=a" (hv_status), ASM_CALL_CONSTRAINT,
> > +		        "+c" (control), "+d" (param1)
> > +		      : "r" (__r8),
> 
> r8 is call-clobbered register, so you should use "+r" (__r8) to properly
> clobber it:

Ah, okay.