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From: Wei Liu <wl@xen.org>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: "Wei Liu" <liuwe@microsoft.com>, "Wei Liu" <wl@xen.org>,
	"Paul Durrant" <paul@xen.org>,
	"Michael Kelley" <mikelley@microsoft.com>,
	"Jan Beulich" <jbeulich@suse.com>,
	"Xen Development List" <xen-devel@lists.xenproject.org>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH v3 2/5] x86/hyperv: provide Hyper-V hypercall functions
Date: Thu, 16 Jan 2020 14:54:49 +0000	[thread overview]
Message-ID: <20200116145449.qt2hjs4drjyxrgkt@debian> (raw)
In-Reply-To: <0020c144-6233-58f7-2f71-c5af03b6b2f8@citrix.com>

On Sun, Jan 05, 2020 at 07:08:28PM +0000, Andrew Cooper wrote:
> 
> > +static inline uint64_t hv_do_hypercall(uint64_t control, paddr_t input, paddr_t output)
> > +{
> > +    uint64_t status;
> > +
> > +    asm volatile ("mov %[output], %%r8\n"
> > +                  "call hv_hypercall_page"
> > +                  : "=a" (status), "+c" (control),
> > +                    "+d" (input) ASM_CALL_CONSTRAINT
> > +                  : [output] "rm" (output)
> > +                  : "cc", "memory", "r8", "r9", "r10", "r11");
> 
> I think you want:
> 
> register unsigned long r8 asm("r8") = output;
> 
> and "+r" (r8) as an output constraint.
> 
> In particular, that doesn't force the compiler to put output into a
> register other than r8 (or worse, spill it to the stack) to have the
> opaque blob of asm move it back into r8.  What it will do in practice is
> cause the compiler to construct output directly in r8.
> 
> As for the other clobbers, I can't find anything at all in the spec
> which even mentions those registers.  There will be a decent improvement
> to code generation if we don't force them to be spilled around a hypercall.

This is actually from Windows 2012 R2's TLFS.

Current version of Hyper-V doesn't clobber those registers anymore. I
think just putting "memory" there should be fine.

Wei.

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  parent reply	other threads:[~2020-01-16 14:55 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-05 16:47 [Xen-devel] [PATCH v3 0/5] More Hyper-V infrastructure Wei Liu
2020-01-05 16:47 ` [Xen-devel] [PATCH v3 1/5] x86/hyperv: setup hypercall page Wei Liu
2020-01-05 17:37   ` Andrew Cooper
2020-01-05 21:45     ` Wei Liu
2020-01-05 21:57       ` Andrew Cooper
2020-01-07 15:42         ` Wei Liu
2020-01-08 17:43           ` Wei Liu
2020-01-08 17:53             ` Andrew Cooper
2020-01-09 13:48               ` Wei Liu
2020-01-05 16:47 ` [Xen-devel] [PATCH v3 2/5] x86/hyperv: provide Hyper-V hypercall functions Wei Liu
2020-01-05 19:08   ` Andrew Cooper
2020-01-05 21:22     ` Wei Liu
2020-01-05 22:06       ` Andrew Cooper
2020-01-07 16:17         ` Wei Liu
2020-01-16 19:14           ` Andrew Cooper
2020-01-16 14:54     ` Wei Liu [this message]
2020-01-06  9:38   ` Jan Beulich
2020-01-07 16:21     ` Wei Liu
2020-01-05 16:47 ` [Xen-devel] [PATCH v3 3/5] x86/hyperv: provide percpu hypercall input page Wei Liu
2020-01-06 10:27   ` Jan Beulich
2020-01-07 16:33     ` Wei Liu
2020-01-07 16:45       ` Michael Kelley
2020-01-08 10:57         ` Jan Beulich
2020-01-07 17:08       ` Jan Beulich
2020-01-07 17:27         ` Wei Liu
2020-01-08 10:55           ` Jan Beulich
2020-01-08 15:54             ` Wei Liu
2020-01-05 16:48 ` [Xen-devel] [PATCH v3 4/5] x86/hyperv: retrieve vp_index from Hyper-V Wei Liu
2020-01-06  9:59   ` Paul Durrant
2020-01-06 10:31   ` Jan Beulich
2020-01-07 16:34     ` Wei Liu
2020-01-05 16:48 ` [Xen-devel] [PATCH v3 5/5] x86/hyperv: setup VP assist page Wei Liu

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