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

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Date:   Tue, 16 Aug 2022 09:53:01 -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:

>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.

Thanks,
Davidlohr

>
>The functionality this is enabling is to be able to instantaneously
>secure erase potentially terabytes of memory at once and the kernel
>needs to be sure that none of the data from before the secure is still
>present in the cache. It is also used when unlocking a memory device
>where speculative reads and firmware accesses could have cached poison
>from before the device was unlocked.
>
>This capability is typically only used once per-boot (for unlock), or
>once per bare metal provisioning event (secure erase), like when handing
>off the system to another tenant. That small scope plus the fact that
>none of this is available to a VM limits the potential damage. So,
>similar to the mitigation we did in [2] that did not kill off wbinvd
>completely, this is limited to specific scenarios and should be disabled
>in any scenario where wbinvd is painful / forbidden.
>
>[1]: 4c6926a23b76 ("acpi/nfit, libnvdimm: Add unlock of nvdimm support for Intel DIMMs")
>[2]: e2efb6359e62 ("ACPICA: Avoid cache flush inside virtual machines")