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

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Date:   Tue, 7 Mar 2017 13:45:07 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Andrew Cooper <andrew.cooper3@...rix.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        "Zhang, Xiong Y" <xiong.y.zhang@...el.com>
Cc:     linux-arch@...r.kernel.org, Juergen Gross <jgross@...e.com>,
        Andi Kleen <ak@...ux.intel.com>, Arnd Bergmann <arnd@...db.de>,
        linux-mm@...ck.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...capital.net>,
        Dave Hansen <dave.hansen@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        xen-devel <xen-devel@...ts.xen.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [Xen-devel] [PATCHv4 18/33] x86/xen: convert __xen_pgd_walk() and
 xen_cleanmfnmap() to support p4d

On 03/07/2017 01:26 PM, Andrew Cooper wrote:
> On 07/03/17 18:18, Boris Ostrovsky wrote:
>>>> 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).
> Wasn't the plan to be able to automatically detect 4 vs 5 level support,
> and cope either way, so distros didn't have to ship two different builds
> of Linux?
>
> If so, all we need to do git things to compile sensibly, and have the PV
> entry code in Linux configure the rest of the kernel appropriately.

I am not aware of any plans but this would obviously be the preferred route.

-boris