linux-kernel - Re: [PATCH 07/11] powerpc/64s: Add support for RFI flush of L1-D cache

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Date:   Wed, 10 Jan 2018 03:05:11 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Joel Stanley <joel@....id.au>
Cc:     linuxppc-dev@...abs.org, Michael Neuling <mikey@...ling.org>,
        peterz@...radead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        npiggin@...il.com, Oliver O'Halloran <oohall@...il.com>,
        Anton Blanchard <anton@...ba.org>,
        Paul Mackerras <paulus@...ba.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 07/11] powerpc/64s: Add support for RFI flush of L1-D cache

Joel Stanley <joel@....id.au> writes:

> On Mon, Jan 8, 2018 at 8:54 AM, Michael Ellerman <mpe@...erman.id.au> wrote:
>> On some CPUs we can prevent the Meltdown vulnerability by flushing the
>> L1-D cache on exit from kernel to user mode, and from hypervisor to
>> guest.
>
> Super minor nitpicks below. Don't let this hold up your work.
>
>> --- a/arch/powerpc/kernel/exceptions-64s.S
>> +++ b/arch/powerpc/kernel/exceptions-64s.S
>> @@ -1449,6 +1449,88 @@ masked_##_H##interrupt:                                  \
>>         b       .;                                      \
>>         MASKED_DEC_HANDLER(_H)
>>
>> +TRAMP_REAL_BEGIN(rfi_flush_fallback)
>> +       SET_SCRATCH0(r13);
>> +       GET_PACA(r13);
>> +       std     r9,PACA_EXRFI+EX_R9(r13)
>> +       std     r10,PACA_EXRFI+EX_R10(r13)
>> +       std     r11,PACA_EXRFI+EX_R11(r13)
>> +       std     r12,PACA_EXRFI+EX_R12(r13)
>> +       std     r8,PACA_EXRFI+EX_R13(r13)
>> +       mfctr   r9
>> +       ld      r10,PACA_RFI_FLUSH_FALLBACK_AREA(r13)
>> +       ld      r11,PACA_L1D_FLUSH_SETS(r13)
>> +       ld      r12,PACA_L1D_FLUSH_CONGRUENCE(r13)
>> +       /*
>> +        * The load adresses are at staggered offsets within cachelines,
>> +        * which suits some pipelines better (on others it should not
>> +        * hurt.
>
> Nit: missing ) on the last line.

Thanks.

>> --- a/arch/powerpc/lib/feature-fixups.c
>> +++ b/arch/powerpc/lib/feature-fixups.c
>> @@ -116,6 +116,47 @@ void do_feature_fixups(unsigned long value, void *fixup_start, void *fixup_end)
>>         }
>>  }
>>
>> +#ifdef CONFIG_PPC_BOOK3S_64
>> +void do_rfi_flush_fixups(enum l1d_flush_type types)
>> +{
>> +       unsigned int instrs[3], *dest;
>> +       long *start, *end;
>> +       int i;
>> +
>> +       start = PTRRELOC(&__start___rfi_flush_fixup),
>> +       end = PTRRELOC(&__stop___rfi_flush_fixup);
>> +
>> +       instrs[0] = 0x60000000; /* nop */
>> +       instrs[1] = 0x60000000; /* nop */
>> +       instrs[2] = 0x60000000; /* nop */
>> +
>> +       if (types & L1D_FLUSH_FALLBACK)
>
> This looked a bit confusing on first read. Do we ever get
> L1D_FLUSH_FALLBACK and the other types? If not, could it be made
> clearer?
>
>   if ( types & L1D_FLUSH_FALLBACK)
>       /* stuff */
>   else
>     /* other types */

Yeah that would make more sense.

I'm going to merge it as-is because that's whats been tested, and given
the time of day I'm likely to break it if I try and refactor it. So we
can clean it up once it's merged.

cheers