linux-kernel - Re: [PATCH 0/6] Boot-time switching between 4- and 5-level paging for 4.15, Part 1

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Date:   Tue, 24 Oct 2017 14:43:11 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     hpa@...or.com
Cc:     Ingo Molnar <mingo@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Borislav Petkov <bp@...e.de>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6] Boot-time switching between 4- and 5-level paging
 for 4.15, Part 1

On Tue, Oct 24, 2017 at 01:32:51PM +0200, hpa@...or.com wrote:
> On October 17, 2017 5:42:41 PM GMT+02:00, "Kirill A. Shutemov" <kirill@...temov.name> wrote:
> >On Tue, Oct 03, 2017 at 11:27:54AM +0300, Kirill A. Shutemov wrote:
> >> On Fri, Sep 29, 2017 at 05:08:15PM +0300, Kirill A. Shutemov wrote:
> >> > The first bunch of patches that prepare kernel to boot-time
> >switching
> >> > between paging modes.
> >> > 
> >> > Please review and consider applying.
> >> 
> >> Ping?
> >
> >Ingo, is there anything I can do to get review easier for you?
> >
> >I hoped to get boot-time switching code into v4.15...
> 
> One issue that has come up with this is what happens if the kernel is
> loaded above 4 GB and we need to switch page table mode.  In that case
> we need enough memory below the 4 GB point to hold a root page table
> (since we can't write the upper half of cr3 outside of 64-bit mode) and
> a handful of instructions.
> 
> We have no real way to know for sure what memory is safe without parsing
> all the memory maps and map out all the data structures that The
> bootloader has left for the kernel.  I'm thinking that the best way to
> deal with this is to add an entry in setup_data to provide a pointers,
> with the kernel header specifying a necessary size and alignment.

I would appreciate your feedback on my take on this:

http://lkml.kernel.org/r/20171020195934.32108-1-kirill.shutemov@linux.intel.com

I don't change boot protocol, but trying to guess the safe spot in the way
similar to what we do for realmode trampoline.

-- 
 Kirill A. Shutemov