linux-kernel - Re: [PATCH v3 08/14] x86, dax, libnvdimm: move wb_cache

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Date:   Wed, 14 Jun 2017 09:49:29 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jan Kara <jack@...e.cz>
Cc:     "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        dm-devel@...hat.com, Matthew Wilcox <mawilcox@...rosoft.com>,
        X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>, Jeff Moyer <jmoyer@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        "Oliver O'Halloran" <oohall@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "H. Peter Anvin" <hpa@...or.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [PATCH v3 08/14] x86, dax, libnvdimm: move wb_cache_pmem() to libnvdimm

On Wed, Jun 14, 2017 at 3:54 AM, Jan Kara <jack@...e.cz> wrote:
> On Fri 09-06-17 13:24:29, Dan Williams wrote:
>> With all calls to this routine re-directed through the pmem driver, we can kill
>> the pmem api indirection. arch_wb_cache_pmem() is now optionally supplied by
>> the arch specific asm/pmem.h.  Same as before, pmem flushing is only defined
>> for x86_64, but it is straightforward to add other archs in the future.
>>
>> Cc: <x86@...nel.org>
>> Cc: Jan Kara <jack@...e.cz>
>> Cc: Jeff Moyer <jmoyer@...hat.com>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: Christoph Hellwig <hch@....de>
>> Cc: "H. Peter Anvin" <hpa@...or.com>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Oliver O'Halloran <oohall@...il.com>
>> Cc: Matthew Wilcox <mawilcox@...rosoft.com>
>> Cc: Ross Zwisler <ross.zwisler@...ux.intel.com>
>> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
>
> Looks good to me. Just one question below...
>
>> -/**
>> - * arch_wb_cache_pmem - write back a cache range with CLWB
>> - * @vaddr:   virtual start address
>> - * @size:    number of bytes to write back
>> - *
>> - * Write back a cache range using the CLWB (cache line write back)
>> - * instruction. Note that @size is internally rounded up to be cache
>> - * line size aligned.
>> - */
>>  static inline void arch_wb_cache_pmem(void *addr, size_t size)
>>  {
>> -     u16 x86_clflush_size = boot_cpu_data.x86_clflush_size;
>> -     unsigned long clflush_mask = x86_clflush_size - 1;
>> -     void *vend = addr + size;
>> -     void *p;
>> -
>> -     for (p = (void *)((unsigned long)addr & ~clflush_mask);
>> -          p < vend; p += x86_clflush_size)
>> -             clwb(p);
>> +     clean_cache_range(addr,size);
>>  }
>
> So this will make compilation break on 32-bit x86 as it does not define
> clean_cache_range(). Do we somewhere force we are on x86_64 when pmem is
> enabled?

Yes, this is enforced by:

    select ARCH_HAS_PMEM_API if X86_64

...in arch/x86/Kconfig. We fallback to a dummy arch_wb_cache_pmem()
implementation and emit this warning for !ARCH_HAS_PMEM_API archs:

    "nd_pmem namespace0.0: unable to guarantee persistence of writes"