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Date:   Tue, 16 Aug 2022 10:52:59 -0700
From:   Davidlohr Bueso <dave@stgolabs.net>
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        linux-cxl@vger.kernel.org, nvdimm@lists.linux.dev,
        bwidawsk@kernel.org, ira.weiny@intel.com, vishal.l.verma@intel.com,
        alison.schofield@intel.com, a.manzanares@samsung.com,
        linux-arch@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
        linux-arm-kernel@lists.infradead.org, bp@alien8.de, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] arch/cacheflush: Introduce flush_all_caches()
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On Tue, 16 Aug 2022, Dan Williams wrote:

>On Tue, Aug 16, 2022 at 10:30 AM Davidlohr Bueso <dave@stgolabs.net> wrote:
>>
>> On Tue, 16 Aug 2022, Dan Williams wrote:
>>
>> >Peter Zijlstra wrote:
>> >> On Mon, Aug 15, 2022 at 09:07:06AM -0700, Davidlohr Bueso wrote:
>> >> > diff --git a/arch/x86/include/asm/cacheflush.h b/arch/x86/include/asm/cacheflush.h
>> >> > index b192d917a6d0..ce2ec9556093 100644
>> >> > --- a/arch/x86/include/asm/cacheflush.h
>> >> > +++ b/arch/x86/include/asm/cacheflush.h
>> >> > @@ -10,4 +10,7 @@
>> >> >
>> >> >  void clflush_cache_range(void *addr, unsigned int size);
>> >> >
>> >> > +#define flush_all_caches() \
>> >> > +  do { wbinvd_on_all_cpus(); } while(0)
>> >> > +
>> >>
>> >> This is horrific... we've done our utmost best to remove all WBINVD
>> >> usage and here you're adding it back in the most horrible form possible
>> >> ?!?
>> >>
>> >> Please don't do this, do *NOT* use WBINVD.
>> >
>> >Unfortunately there are a few good options here, and the changelog did
>> >not make clear that this is continuing legacy [1], not adding new wbinvd
>> >usage.
>>
>> While I was hoping that it was obvious from the intel.c changes that this
>> was not a new wbinvd, I can certainly improve the changelog with the below.
>
>I also think this cache_flush_region() API wants a prominent comment
>clarifying the limited applicability of this API. I.e. that it is not
>for general purpose usage, not for VMs, and only for select bare metal
>scenarios that instantaneously invalidate wide swaths of memory.
>Otherwise, I can now see how this looks like a potentially scary
>expansion of the usage of wbinvd.

Sure.

Also, in the future we might be able to bypass this hammer in the presence
of persistent cpu caches.

Thanks,
Davidlohr