From: Jan Beulich <jbeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Xen-devel <xen-devel@lists.xenproject.org>,
"Wei Liu" <wl@xen.org>, "Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH 3/4] x86/boot: Create the l2_xenmap[] mappings dynamically
Date: Wed, 15 Jan 2020 10:23:32 +0100 [thread overview]
Message-ID: <ce1d41a1-6865-2a0d-9aa0-30fc82cad557@suse.com> (raw)
In-Reply-To: <ecf2a65e-68ea-63c0-0fb3-576104d7e202@citrix.com>
On 14.01.2020 20:31, Andrew Cooper wrote:
> On 14/01/2020 16:45, Jan Beulich wrote:
>> On 13.01.2020 18:50, Andrew Cooper wrote:
>>> --- a/xen/arch/x86/boot/head.S
>>> +++ b/xen/arch/x86/boot/head.S
>>> @@ -668,6 +668,20 @@ trampoline_setup:
>>> add %esi,sym_fs(__page_tables_start)-8(,%ecx,8)
>>> 2: loop 1b
>>>
>>> + /* Map Xen into the higher mappings using 2M superpages. */
>>> + lea _PAGE_PSE + PAGE_HYPERVISOR_RWX + sym_esi(_start), %eax
>>> + mov $sym_offs(_start), %ecx /* %eax = PTE to write */
>> The comment is on the wrong line, isn't it? Perhaps
>>
>> lea _PAGE_PSE + PAGE_HYPERVISOR_RWX + sym_esi(_start), \
>> %eax /* %eax = PTE to write */
>>
>> ?
>
> That is why the comment had the register name, rather than trying to
> claim that $sym_offs(_start) was the PTE to write.
>
> I didn't really think splitting the lea like that across 2 lines was
> better than this.
>
> How about /* %eax = PTE to write ^ */ which will point properly at
> %eax?
Fine with me; I assume you mean this to go on a separate line?
>>> --- a/xen/arch/x86/efi/efi-boot.h
>>> +++ b/xen/arch/x86/efi/efi-boot.h
>>> @@ -585,6 +585,20 @@ static void __init efi_arch_memory_setup(void)
>>> if ( !efi_enabled(EFI_LOADER) )
>>> return;
>>>
>>> + /*
>>> + * Map Xen into the higher mappings, using 2M superpages.
>>> + *
>>> + * NB: We are currently in physical mode, so a RIP-relative relocation
>>> + * against _start/_end gets their position as placed by the bootloader,
>>> + * not as expected in the final build. This has arbitrary 2M alignment,
>>> + * so subtract xen_phys_start to get the appropriate slots in l2_xenmap[].
>>> + */
>> It may just be a language issue, but I'm struggling with the
>> "arbitrary" here. Is this in any way related to the
>> --section-alignment=0x200000 option we pass to the linker (where
>> the value isn't arbitrary at all)?
>
> So this is the bug I spent ages trying to figure out console logging for.
>
> The naive version of this loop (pre subtraction) ended up initialising
> slots 173...177 which, when highlighted like that, is obviously why Xen
> triple faulted when switching to the high mappings.
>
> The point I'm trying to make is that l2_table_offset(_start) ends up
> being junk because it is a rip-relative address and we're not running at
> our linked address. (It is in fact our physical position in memory's 2M
> slot, modulo 512).
>
> Subtracting xen_phys_start gets the number back into the same alias
> which all the 32bit head.S code relies on, and gives us a sensible
> sequence of slots starting from 1.
Thanks for the explanation. What I'm still unclear about is this use
of "arbitrary", though. Looking at it again I guess I'm also
struggling to understand what "This" at the beginning of the sentence
refers to.
Jan
_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel
next prev parent reply other threads:[~2020-01-15 9:24 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-13 17:50 [Xen-devel] [PATCH 0/4] x86: Remove 16M total-size restriction Andrew Cooper
2020-01-13 17:50 ` [Xen-devel] [PATCH 1/4] x86/boot: Rename l?_identmap to l?_directmap Andrew Cooper
2020-01-14 16:16 ` Jan Beulich
2020-01-13 17:50 ` [Xen-devel] [PATCH 2/4] x86/page: Remove bifrucated PAGE_HYPERVISOR constant Andrew Cooper
2020-01-13 17:50 ` [Xen-devel] [PATCH 2/4] x86/page: Remove bifurcated " Andrew Cooper
2020-01-14 16:25 ` Jan Beulich
2020-01-15 12:53 ` Andrew Cooper
2020-01-15 13:07 ` Jan Beulich
2020-01-15 14:08 ` [Xen-devel] [PATCH v2 " Andrew Cooper
2020-01-16 9:46 ` Jan Beulich
2020-01-13 17:50 ` [Xen-devel] [PATCH 3/4] x86/boot: Create the l2_xenmap[] mappings dynamically Andrew Cooper
2020-01-14 16:45 ` Jan Beulich
2020-01-14 19:31 ` Andrew Cooper
2020-01-15 9:23 ` Jan Beulich [this message]
2020-01-16 19:41 ` Andrew Cooper
2020-01-13 17:50 ` [Xen-devel] [PATCH 4/4] x86/boot: Size the boot/directmap " Andrew Cooper
2020-01-14 17:02 ` Jan Beulich
2020-01-14 17:27 ` Andrew Cooper
2020-01-15 9:40 ` Jan Beulich
2020-01-15 10:21 ` Jan Beulich
2020-01-14 13:03 ` [Xen-devel] [PATCH 0/4] x86: Remove 16M total-size restriction Andrew Cooper
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=ce1d41a1-6865-2a0d-9aa0-30fc82cad557@suse.com \
--to=jbeulich@suse.com \
--cc=andrew.cooper3@citrix.com \
--cc=roger.pau@citrix.com \
--cc=wl@xen.org \
--cc=xen-devel@lists.xenproject.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).