Re: [PATCH v4 04/17] x86/acrn: Introduce hypercall interfaces

From: Arvind Sankar <nivedita@alum.mit.edu>
To: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Segher Boessenkool <segher@kernel.crashing.org>,
	Dave Hansen <dave.hansen@intel.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	shuo.a.liu@intel.com, LKML <linux-kernel@vger.kernel.org>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, "H . Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	Yu Wang <yu1.wang@intel.com>,
	Reinette Chatre <reinette.chatre@intel.com>,
	Yakui Zhao <yakui.zhao@intel.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Fengwei Yin <fengwei.yin@intel.com>,
	Zhi Wang <zhi.a.wang@intel.com>,
	Zhenyu Wang <zhenyuw@linux.intel.com>
Subject: Re: [PATCH v4 04/17] x86/acrn: Introduce hypercall interfaces
Date: Wed, 30 Sep 2020 16:01:18 -0400	[thread overview]
Message-ID: <20200930200118.GA3185118@rani.riverdale.lan> (raw)
In-Reply-To: <20200930195915.GA3180913@rani.riverdale.lan>

On Wed, Sep 30, 2020 at 03:59:15PM -0400, Arvind Sankar wrote:
> On Wed, Sep 30, 2020 at 12:14:03PM -0700, Nick Desaulniers wrote:
> > On Wed, Sep 30, 2020 at 10:13 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > On Wed, Sep 30, 2020 at 11:10:36AM -0500, Segher Boessenkool wrote:
> > >
> > > > Since this variable is a local register asm, on entry to the asm the
> > > > compiler guarantees that the value lives in the assigned register (the
> > > > "r8" hardware register in this case).  This all works completely fine.
> > > > This is the only guaranteed behaviour for local register asm (well,
> > > > together with analogous behaviour for outputs).
> 
> How strict is the guarantee? This is an inline function -- could the
> compiler decide to reorder some other code in between the r8 assignment
> and the asm statement when it gets inlined?
> 
> > >
> > > Right, that's what they're trying to achieve. The hypervisor calling
> > > convention needs that variable in %r8 (which is somewhat unfortunate).
> > >
> > > AFAIK this is the first such use in the kernel, but at least the gcc-4.9
> > > (our oldest supported version) claims to support this.
> > >
> > > So now we need to know if clang will actually do this too..
> > 
> > Does clang support register local storage? Let's use godbolt.org to find out:
> > https://godbolt.org/z/YM45W5
> > Looks like yes. You can even check different GCC versions via the
> > dropdown in the top right.
> > 
> > The -ffixed-* flags are less well supported in Clang; they need to be
> > reimplemented on a per-backend basis. aarch64 is relatively well
> > supported, but other arches not so much IME.
> > 
> > Do we need register local storage here?
> > 
> > static inline long bar(unsigned long hcall_id)
> > {
> >   long result;
> >   asm volatile("movl %1, %%r8d\n\t"
> >   "vmcall\n\t"
> >     : "=a" (result)
> >     : "ir" (hcall_id)
> >     : );
> >   return result;
> > }
> 
> This seems more robust, though you probably need an r8 clobber in there?
> Is hcall_id actually just 32 bits or can it be >=2^32?

Also, I think you need memory clobbers for all of these in either case, no?