linux-kernel - Re: [PATCH v2 0/2] add support for new persistent memory instructions

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Date:	Sat, 24 Jan 2015 12:14:30 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	"H. Peter Anvin" <h.peter.anvin@...el.com>
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 0/2] add support for new persistent memory instructions

On Fri, Jan 23, 2015 at 03:03:41PM -0800, H. Peter Anvin wrote:
> For the specific case of CLWB, we can use an "m" input rather than a
> "+m" output, simply because CLWB (or CLFLUSH* used as a standin for CLWB
> doesn't need to be ordered with respect to loads (whereas CLFLUSH* do).

Well, we could do something like:

        volatile struct { char x[64]; } *p = __p;

        if (static_cpu_has(X86_FEATURE_CLWB))
                asm volatile(".byte 0x66,0x0f,0xae,0x30" :: "m" (*p), "a" (p));
        else
                asm volatile(ALTERNATIVE(
                        ".byte 0x3e; clflush (%[pax])",
                        ".byte 0x66; clflush (%[pax])", /* clflushopt (%%rax) */
                        X86_FEATURE_CLFLUSHOPT)
                        : [p] "+m" (*p)
                        : [pax] "a" (p));

which would simplify the alternative macro too.

Generated asm looks ok to me (my objdump doesn't know CLWB yet :)):

0000000000000aa0 <myclflush>:
 aa0:   55                      push   %rbp
 aa1:   48 89 e5                mov    %rsp,%rbp
 aa4:   eb 0a                   jmp    ab0 <myclflush+0x10>
 aa6:   48 89 f8                mov    %rdi,%rax
 aa9:   66 0f ae 30             data16 xsaveopt (%rax)
 aad:   5d                      pop    %rbp
 aae:   c3                      retq
 aaf:   90                      nop
 ab0:   48 89 f8                mov    %rdi,%rax
 ab3:   3e 0f ae 38             clflush %ds:(%rax)
 ab7:   5d                      pop    %rbp
 ab8:   c3                      retq

> Should we use an SFENCE as a standin if pcommit is unavailable, in case
> we end up using CLFLUSHOPT?

Btw, is PCOMMIT a lightweight SFENCE for this persistent memory aspect
to make sure stuff has become persistent after executing it? But not all
stuff like SFENCE so SFENCE is the bigger hammer?

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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