Re: [PATCH v3 03/18] x86/tsc/paravirt: Remove the read_tsc and read_tscp paravirt hooks

From: Andy Lutomirski <luto@amacapital.net>
To: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	John Stultz <john.stultz@linaro.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Len Brown <lenb@kernel.org>, Huang Rui <ray.huang@amd.com>,
	Denys Vlasenko <dvlasenk@redhat.com>,
	kvm list <kvm@vger.kernel.org>,
	Ralf Baechle <ralf@linux-mips.org>
Subject: Re: [PATCH v3 03/18] x86/tsc/paravirt: Remove the read_tsc and read_tscp paravirt hooks
Date: Fri, 19 Jun 2015 09:14:14 -0700	[thread overview]
Message-ID: <CALCETrX6eF69da2qchpS=G=P25QLgGsZXYTo=QcTocSxdbLH9A@mail.gmail.com> (raw)
In-Reply-To: <20150619153201.GF12979@pd.tnic>

On Fri, Jun 19, 2015 at 8:32 AM, Borislav Petkov <bp@suse.de> wrote:
> On Tue, Jun 16, 2015 at 05:35:51PM -0700, Andy Lutomirski wrote:
>> We've had read_tsc and read_tscp paravirt hooks since the very
>> beginning of paravirt, i.e., d3561b7fa0fb ("[PATCH] paravirt: header
>> and stubs for paravirtualisation").  AFAICT the only paravirt guest
>> implementation that ever replaced these calls was vmware, and it's
>> gone.  Arguably even vmware shouldn't have hooked rdtsc -- we fully
>> support systems that don't have a TSC at all, so there's no point
>> for a paravirt implementation to pretend that we have a TSC but to
>> replace it.
>>
>> I also doubt that these hooks actually worked.  Calls to rdtscl and
>> rdtscll, which respected the hooks, were used seemingly
>> interchangeably with native_read_tsc, which did not.
>>
>> Just remove them.  If anyone ever needs them again, they can try
>> to make a case for why they need them.
>>
>> Before, on a paravirt config:
>>    text          data     bss     dec     hex filename
>> 13426505      1827056 14508032        29761593        1c62039 vmlinux
>>
>> After:
>>    text          data     bss     dec     hex filename
>> 13426617      1827056 14508032        29761705        1c620a9 vmlinux
>
> Those look swapped. I mean, you're removing a bunch of stuff and text
> grew?!

Trying again.  The config is different, so the numbers won't match.

Before:
   text    data     bss     dec     hex filename
12618257        1816384 1093632 15528273         ecf151 vmlinux

After:
   text    data     bss     dec     hex filename
12617207        1816384 1093632 15527223         eced37 vmlinux

So, yes, I have it backwards.  There's some penalty here because this
patch also causes rdtsc to be inlined on paravirt kernels (it was
patched as a call instead of an inline instruction sequence), so each
call site pays for the shift, the or, and the pointless zero
extension.

With the "unsigned long" patch, it's:

   text    data     bss     dec     hex filename
12617002        1816384 1093632 15527018         ecec6a vmlinux

Want to fix up the commit message?  It seems silly to re-send the
whole series for this.

--Andy

>
>> Signed-off-by: Andy Lutomirski <luto@kernel.org>
>> ---
>>  arch/x86/include/asm/msr.h            | 16 ++++++++--------
>>  arch/x86/include/asm/paravirt.h       | 34 ----------------------------------
>>  arch/x86/include/asm/paravirt_types.h |  2 --
>>  arch/x86/kernel/paravirt.c            |  2 --
>>  arch/x86/kernel/paravirt_patch_32.c   |  2 --
>>  arch/x86/xen/enlighten.c              |  3 ---
>>  6 files changed, 8 insertions(+), 51 deletions(-)
>
> --
> Regards/Gruss,
>     Boris.
>
> ECO tip #101: Trim your mails when you reply.
> --

-- 
Andy Lutomirski
AMA Capital Management, LLC