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Date: Fri, 17 Mar 2017 11:17:37 +0100
From: Borislav Petkov <bp@suse.de>
Subject: Re: [RFC PATCH v2 14/32] x86: mm: Provide support to use memblock
 when spliting large pages
Message-ID: <20170317101737.icdois7sdmtutt6b@pd.tnic>
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>, simon.guinot@sequanux.org, linux-efi@vger.kernel.org, kvm@vger.kernel.org, rkrcmar@redhat.com, matt@codeblueprint.co.uk, linux-pci@vger.kernel.org, linus.walleij@linaro.org, gary.hook@amd.com, linux-mm@kvack.org, paul.gortmaker@windriver.com, hpa@zytor.com, cl@linux.com, dan.j.williams@intel.com, aarcange@redhat.com, sfr@canb.auug.org.au, andriy.shevchenko@linux.intel.com, herbert@gondor.apana.org.au, bhe@redhat.com, xemul@parallels.com, joro@8bytes.org, x86@kernel.org, peterz@infradead.org, piotr.luc@intel.com, mingo@redhat.com, msalter@redhat.com, ross.zwisler@linux.intel.com, dyoung@redhat.com, thomas.lendacky@amd.com, jroedel@suse.de, keescook@chromium.org, arnd@arndb.de, toshi.kani@hpe.com, mathieu.desnoyers@efficios.com, luto@kernel.org, devel@linuxdriverproject.org, bhelgaas@google.com, tglx@linutronix.de, mchehab@kernel.org, iamjoonsoo.kim@lge.com, labbott@fedo.suse.de

On Thu, Mar 16, 2017 at 11:25:36PM +0100, Paolo Bonzini wrote:
> The kvmclock memory is initially zero so there is no need for the
> hypervisor to allocate anything; the point of these patches is just to
> access the data in a natural way from Linux source code.

I realize that.

> I also don't really like the patch as is (plus it fails modpost), but
> IMO reusing __change_page_attr and __split_large_page is the right thing
> to do.

Right, so teaching pageattr.c about memblock could theoretically come
around and bite us later when a page allocated with memblock gets freed
with free_page().

And looking at this more, we have all this kernel pagetable preparation
code down the init_mem_mapping() call and the pagetable setup in
arch/x86/mm/init_{32,64}.c

And that code even does some basic page splitting. Oh and it uses
alloc_low_pages() which knows whether to do memblock reservation or the
common __get_free_pages() when slabs are up.

So what would be much cleaner, IMHO, is if one would reuse that code to
change init_mm.pgd mappings early without copying pageattr.c.

init_mem_mapping() gets called before kvm_guest_init() in setup_arch()
so the guest would simply fixup its pagetable right there.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix ImendA?rffer, Jane Smithard, Graham Norton, HRB 21284 (AG NA 1/4 rnberg)
-- 

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