linux-kernel - Re: [PATCH -next] memregion: Add arch_flush

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Date:   Wed, 7 Sep 2022 09:52:17 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Davidlohr Bueso <dave@...olabs.net>, Borislav Petkov <bp@...en8.de>
CC:     <dan.j.williams@...el.com>, <x86@...nel.org>,
        <nvdimm@...ts.linux.dev>, <linux-cxl@...r.kernel.org>,
        <peterz@...radead.org>, <akpm@...ux-foundation.org>,
        <dave.jiang@...el.com>, <Jonathan.Cameron@...wei.com>,
        <vishal.l.verma@...el.com>, <ira.weiny@...el.com>,
        <a.manzanares@...sung.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] memregion: Add arch_flush_memregion() interface

Davidlohr Bueso wrote:
> On Wed, 07 Sep 2022, Borislav Petkov wrote:
> 
> >On Mon, Aug 29, 2022 at 02:29:18PM -0700, Davidlohr Bueso wrote:
> >> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> >> index 1abd5438f126..18463cb704fb 100644
> >> --- a/arch/x86/mm/pat/set_memory.c
> >> +++ b/arch/x86/mm/pat/set_memory.c
> >> @@ -330,6 +330,20 @@ void arch_invalidate_pmem(void *addr, size_t size)
> >>  EXPORT_SYMBOL_GPL(arch_invalidate_pmem);
> >>  #endif
> >>
> >> +#ifdef CONFIG_ARCH_HAS_MEMREGION_INVALIDATE
> >> +bool arch_has_flush_memregion(void)
> >> +{
> >> +	return !cpu_feature_enabled(X86_FEATURE_HYPERVISOR);
> >
> >This looks really weird. Why does this need to care about HV at all?
> 
> So the context here is:
> 
> e2efb6359e62 ("ACPICA: Avoid cache flush inside virtual machines")
> 
> >
> >Does that nfit stuff even run in guests?
> 
> No, nor does cxl. This was mostly in general a precautionary check such
> that the api is unavailable in VMs.

To be clear nfit stuff and CXL does run in guests, but they do not
support secure-erase in a guest.

However, the QEMU CXL enabling is building the ability to do *guest
physical* address space management, but in that case the driver can be
paravirtualized to realize that it is not managing host-physical address
space and does not need to flush caches. That will need some indicator
to differentiate virtual CXL memory expanders from assigned devices. Is
there such a thing as a PCIe-virtio extended capability to differentiate
physical vs emulated devices?