linux-kernel - Re: [PATCHv4 18/33] x86/xen: convert __xen_pgd_walk() and xen

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Date:   Tue, 7 Mar 2017 13:18:17 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        "Zhang, Xiong Y" <xiong.y.zhang@...el.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Juergen Gross <jgross@...e.com>,
        xen-devel <xen-devel@...ts.xen.org>
Subject: Re: [PATCHv4 18/33] x86/xen: convert __xen_pgd_walk() and
 xen_cleanmfnmap() to support p4d

>> 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