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Subject: Re: [PATCHv4 18/33] x86/xen: convert __xen_pgd_walk() and
 xen_cleanmfnmap() to support p4d
To: "Kirill A. Shutemov" <kirill@shutemov.name>,
        "Zhang, Xiong Y" <xiong.y.zhang@intel.com>
References: <20170306135357.3124-1-kirill.shutemov@linux.intel.com>
 <20170306135357.3124-19-kirill.shutemov@linux.intel.com>
 <ab2868ea-1dd1-d51b-4c5a-921ef5c9a427@oracle.com>
 <20170307130009.GA2154@node>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>, x86@kernel.org,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>, "H. Peter Anvin" <hpa@zytor.com>,
        Andi Kleen <ak@linux.intel.com>, Dave Hansen <dave.hansen@intel.com>,
        Andy Lutomirski <luto@amacapital.net>, linux-arch@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Juergen Gross <jgross@suse.com>, xen-devel <xen-devel@lists.xen.org>
From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Message-ID: <8bd7d5b7-7a22-a0a2-8eff-e909a1c6783e@oracle.com>
Date: Tue, 7 Mar 2017 13:18:17 -0500
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>> Don't we need to pass vaddr down to all routines so that they select
>> appropriate tables? You seem to always be choosing the first one.
> IIUC, we clear whole page table subtree covered by one pgd entry.
> So, no, there's no need to pass vaddr down. Just pointer to page table
> entry is enough.
>
> But I know virtually nothing about Xen. Please re-check my reasoning.

Yes, we effectively remove the whole page table for vaddr so I guess
it's OK.

>
> I would also appreciate help with getting x86 Xen code work with 5-level
> paging enabled. For now I make CONFIG_XEN dependent on !CONFIG_X86_5LEVEL.

Hmmm... that's a problem since this requires changes in the hypervisor
and even if/when these changes are made older version of hypervisor
still will not be able to run those guests.

This affects only PV guests and there is a series under review that
provides clean code separation with CONFIG_XEN_PV but because, for
example, dom0 (Xen control domain) is PV this will significantly limit
availability of dom0-capable kernels (because I assume distros will want
to have CONFIG_X86_5LEVEL).


>
> Fixup:

Yes, that works. (But then it worked even without this change because
problems caused by missing the flush would be intermittent. And a joy to
debug).

-boris