From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jan Beulich <jbeulich@suse.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 3/2] x86/desc: Drop __HYPERVISOR_CS32
Date: Fri, 9 Aug 2019 16:36:22 +0100 [thread overview]
Message-ID: <eb6e00a0-2937-953e-a57b-c46ab45dfd5a@citrix.com> (raw)
In-Reply-To: <cf80bbdd-bddb-fb8f-f49b-1ef205422f67@suse.com>
On 09/08/2019 13:50, Jan Beulich wrote:
> On 09.08.2019 14:39, Andrew Cooper wrote:
>> Xen, being 64bit only these days, has no use for a 32bit Ring 0 code
>> segment.
>>
>> Delete __HYPERVISOR_CS32 and remove it from the GDTs. Also delete
>> __HYPERVISOR_CS64 and use __HYPERVISOR_CS uniformly.
>
> Long, long ago we've been considering doing this. Agreed,
> nothing has surfaced to actually use it, but I wouldn't
> subscribe to "has no use": We will need it if we ever want
> to be able to run on 32-bit EFI _and_ invoke runtime
> services there. Back then through the consideration against
> dropping it was that we may want to invoke 32-bit BIOS
> services (PCI, VGA) from the hypervisor.
I hadn't realised these had even been considered in the past. I don't
think either of these are likely to happen now.
As for the text, Xen really does have no users of a 32bit R0 code
segment, and the statement does not preclude the fact that there may be
legitimate uses for CS32.
Would you be happier with "days, and does not use a 32bit Ring 0 code
segment." ?
There is specifically a good reason for taking it out (given that it
isn't used), to avoid cascade breakage from a stray far jump which
happens to start executing code in the wrong mode.
~Andrew
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next prev parent reply other threads:[~2019-08-09 15:36 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-09 10:34 [Xen-devel] [PATCH v2 0/2] x86/boot: cleanup Jan Beulich
2019-08-09 10:38 ` [Xen-devel] [PATCH v2 1/2] x86: define a few selector values Jan Beulich
2019-08-09 11:50 ` Andrew Cooper
2019-08-09 12:35 ` Jan Beulich
2019-08-23 2:38 ` Tian, Kevin
2019-08-09 10:40 ` [Xen-devel] [PATCH v2 2/2] x86/desc: Build boot_{, compat_}gdt[] in C Jan Beulich
2019-08-09 12:19 ` Andrew Cooper
2019-08-09 12:43 ` Jan Beulich
2019-08-09 13:07 ` Andrew Cooper
2019-08-09 13:18 ` Jan Beulich
2019-08-09 15:25 ` Andrew Cooper
2019-08-12 7:32 ` Jan Beulich
2019-08-12 10:36 ` Andrew Cooper
2019-08-09 10:41 ` [Xen-devel] [PATCH v2 0/2] x86/boot: cleanup Jan Beulich
2019-08-09 12:39 ` [Xen-devel] [PATCH 3/2] x86/desc: Drop __HYPERVISOR_CS32 Andrew Cooper
2019-08-09 12:50 ` Jan Beulich
2019-08-09 15:36 ` Andrew Cooper [this message]
2019-08-09 15:52 ` Jan Beulich
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