linux-kernel - Re: [PATCH] arch/cacheflush: Introduce flush_all

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Message-ID: <20220816175259.o5h5wv23rs2bvcu6@offworld>
Date:   Tue, 16 Aug 2022 10:52:59 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Dave Jiang <dave.jiang@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        linux-cxl@...r.kernel.org, nvdimm@...ts.linux.dev,
        bwidawsk@...nel.org, ira.weiny@...el.com, vishal.l.verma@...el.com,
        alison.schofield@...el.com, a.manzanares@...sung.com,
        linux-arch@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        linux-arm-kernel@...ts.infradead.org, bp@...en8.de, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arch/cacheflush: Introduce flush_all_caches()

On Tue, 16 Aug 2022, Dan Williams wrote:

>On Tue, Aug 16, 2022 at 10:30 AM Davidlohr Bueso <dave@...olabs.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