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From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Wei Liu <wl@xen.org>
Cc: "Wei Liu" <liuwe@microsoft.com>, "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 19:14:34 +0000
Message-ID: <a42bdfcf-cbb8-56d9-1155-79ae0531001a@citrix.com> (raw)
In-Reply-To: <20200107161733.i3hedh2wcoijcglo@debian>

On 07/01/2020 16:17, Wei Liu wrote:
> On Sun, Jan 05, 2020 at 10:06:08PM +0000, Andrew Cooper wrote:
>> On 05/01/2020 21:22, Wei Liu wrote:
>>> 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.
>>> Although it is named `output`, it is really just an input parameter from
>>> Hyper-V's PoV.
>> Yes, but it is also clobbered.
>>
>> This is an awkward corner case of gnu inline assembly.
>>
>> It is not permitted to have a clobber list overlap with any input/output
>> operations, and because r8 doesn't have a unique letter, you can't do
>> the usual trick of "=r8" (discard) : "r8" (input).
>>
>> The only available option is to mark it as read and written (which is
>> "+r" in the output list), and not use the C variable r8 at any point later.
> But r8 is only listed in clobber list, so it certainly doesn't overlap
> with any input register. I fail to see what the bug (if there is any) is
> here.

%r8 is an input parameter.  You have "mov %[output], %%r8" in the asm.

The way this is written causes the compiler to construct %[output] in a
register, pass it in via the sole input parameter, and behind the scenes
move it into %r8.

There is a small chance of the emitted asm being "mov %r8, %r8", and a
much larger chance of the compiler being forced to spill a different
register when it could have used %r8 in the first place.

> I think what you're asking for here is an optimisation. Is that correct?
> I don't mind changing the code. What I need is clarification here.

I'm on the fence as to whether to put in the category of optimisation,
or buggy asm.

I think the generated code will DTRT, but it is a suspicious looking
piece of assembly, so "optimisation" I guess?

~Andrew

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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 [this message]
2020-01-16 14:54     ` Wei Liu
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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