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

Andy Lutomirski
2015-06-19T16:14:45Z

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
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