linux-kernel - Re: [PATCH] x86/paravirt: Add missing noinstr to arch

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Date:   Wed, 5 Aug 2020 16:17:09 +0200
From:   peterz@...radead.org
To:     Marco Elver <elver@...gle.com>
Cc:     bp@...en8.de, dave.hansen@...ux.intel.com, fenghua.yu@...el.com,
        hpa@...or.com, linux-kernel@...r.kernel.org, mingo@...hat.com,
        syzkaller-bugs@...glegroups.com, tglx@...utronix.de,
        tony.luck@...el.com, x86@...nel.org, yu-cheng.yu@...el.com,
        jgross@...e.com, sdeep@...are.com,
        virtualization@...ts.linux-foundation.org,
        kasan-dev@...glegroups.com,
        syzbot <syzbot+8db9e1ecde74e590a657@...kaller.appspotmail.com>
Subject: Re: [PATCH] x86/paravirt: Add missing noinstr to arch_local*()
 helpers

On Wed, Aug 05, 2020 at 04:12:37PM +0200, peterz@...radead.org wrote:
> On Wed, Aug 05, 2020 at 03:59:40PM +0200, Marco Elver wrote:
> > On Wed, Aug 05, 2020 at 03:42PM +0200, peterz@...radead.org wrote:
> 
> > > Shouldn't we __always_inline those? They're going to be really small.
> > 
> > I can send a v2, and you can choose. For reference, though:
> > 
> > 	ffffffff86271ee0 <arch_local_save_flags>:
> > 	ffffffff86271ee0:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
> > 	ffffffff86271ee5:       48 83 3d 43 87 e4 01    cmpq   $0x0,0x1e48743(%rip)        # ffffffff880ba630 <pv_ops+0x120>
> > 	ffffffff86271eec:       00
> > 	ffffffff86271eed:       74 0d                   je     ffffffff86271efc <arch_local_save_flags+0x1c>
> > 	ffffffff86271eef:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
> > 	ffffffff86271ef4:       ff 14 25 30 a6 0b 88    callq  *0xffffffff880ba630
> > 	ffffffff86271efb:       c3                      retq
> > 	ffffffff86271efc:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
> > 	ffffffff86271f01:       0f 0b                   ud2
> 
> > 	ffffffff86271a90 <arch_local_irq_restore>:
> > 	ffffffff86271a90:       53                      push   %rbx
> > 	ffffffff86271a91:       48 89 fb                mov    %rdi,%rbx
> > 	ffffffff86271a94:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
> > 	ffffffff86271a99:       48 83 3d 97 8b e4 01    cmpq   $0x0,0x1e48b97(%rip)        # ffffffff880ba638 <pv_ops+0x128>
> > 	ffffffff86271aa0:       00
> > 	ffffffff86271aa1:       74 11                   je     ffffffff86271ab4 <arch_local_irq_restore+0x24>
> > 	ffffffff86271aa3:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
> > 	ffffffff86271aa8:       48 89 df                mov    %rbx,%rdi
> > 	ffffffff86271aab:       ff 14 25 38 a6 0b 88    callq  *0xffffffff880ba638
> > 	ffffffff86271ab2:       5b                      pop    %rbx
> > 	ffffffff86271ab3:       c3                      retq
> > 	ffffffff86271ab4:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
> > 	ffffffff86271ab9:       0f 0b                   ud2
> 
> 
> Blergh, that's abysmall. In part I suspect because you have
> CONFIG_PARAVIRT_DEBUG, let me try and untangle that PV macro maze.

Yeah, look here:

0000 0000000000462149 <arch_local_save_flags>:
0000   462149:  ff 14 25 00 00 00 00    callq  *0x0
0003                    46214c: R_X86_64_32S    pv_ops+0x120
0007   462150:  c3                      retq


That's exactly what I was expecting.