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From: "Jan Beulich" <JBeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Julien Grall <julien.grall@arm.com>,
	Paul Durrant <paul.durrant@citrix.com>, Tim Deegan <tim@xen.org>,
	Xen-devel <xen-devel@lists.xen.org>
Subject: Re: [PATCH for 4.9 3/6] x86/hvm: Fix segmentation logic for system segments
Date: Tue, 04 Apr 2017 04:32:28 -0600	[thread overview]
Message-ID: <58E3925C020000780014C85E@prv-mh.provo.novell.com> (raw)
In-Reply-To: <8d4daa75-4312-4ab3-1731-c616ace96a06@citrix.com>

>>> On 04.04.17 at 12:18, <andrew.cooper3@citrix.com> wrote:
> On 03/04/17 18:37, Andrew Cooper wrote:
>>>> Without this fix, implicit accesses to system segments in a
>>>> compatibility mode segment will truncate the resulting linear address as
>>>> part of performing the segmentation calculations, which is obviously not
>>>> how real hardware behaves.
>>> I understand this. But things are a little more complicated. Just
>>> extend your line of thinking regarding IDTR/GDTR to LDTR and
>>> TR: Above you suggest that the former two get loaded in a fully
>>> 32-bit mode compatible way. LTR and LLDT (as well as LAR and
>>> LSL), however, access a descriptor table. 32-bit code would
>>> expect an 8-byte descriptor to be read. Is compat mode code
>>> then not allowed to make the same assumption?
>> Hmm - the wording of LTR/LLDT in both manuals states 64bit mode, not
>> long mode, so there is a decent chance that the compat behaviour is
>> identical.  Let me experiment.
> 
> In a compat mode segment, lldt/ltr operates almost identically to
> protected mode.  They read 8-byte entries, and zero extends the base
> field when loading the result into the segment cache.  However, in
> compatibility mode, they are still subject to long mode restrictions. 
> In particular, you can't attempt to load a 16bit TSS while in a compat
> mode segment.

Interesting, and sort of unexpected. Besides this likely meaning we
need to further adjust the emulator, this raises a question on call
gates then: What factor is it which determines whether a call gate
is an 8- or 16-byte one? Is this perhaps dependent on the L bit of
the code segment descriptor referred to by the gate's code selector?

Jan


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  reply	other threads:[~2017-04-04 10:32 UTC|newest]

Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-03-31 19:50 [PATCH for 4.9 0/6] x86/emul: Fixes Andrew Cooper
2017-03-31 19:50 ` [PATCH for 4.9 1/6] x86/hvm: Correct some address space terminology Andrew Cooper
2017-04-03  8:24   ` Paul Durrant
2017-04-03  8:24   ` Jan Beulich
2017-04-03 10:19     ` Andrew Cooper
2017-04-03 10:29       ` Jan Beulich
2017-03-31 19:50 ` [PATCH for 4.9 2/6] x86/hvm: Correct long mode predicate Andrew Cooper
2017-04-03  8:26   ` Paul Durrant
2017-04-03  8:30   ` Jan Beulich
2017-04-03  8:50   ` George Dunlap
2017-04-05  7:08   ` Tian, Kevin
2017-03-31 19:50 ` [PATCH for 4.9 3/6] x86/hvm: Fix segmentation logic for system segments Andrew Cooper
2017-04-03  8:31   ` Paul Durrant
2017-04-03  9:13   ` Jan Beulich
2017-04-03 14:27     ` Andrew Cooper
2017-04-03 15:07       ` Jan Beulich
2017-04-03 15:42         ` Andrew Cooper
2017-04-03 16:08           ` Jan Beulich
2017-04-03 17:37             ` Andrew Cooper
2017-04-04 10:18               ` Andrew Cooper
2017-04-04 10:32                 ` Jan Beulich [this message]
2017-03-31 19:50 ` [PATCH for 4.9 4/6] x86/svm: Introduce svm_emul_swint_injection() Andrew Cooper
2017-04-03  9:30   ` Jan Beulich
2017-04-03 14:04   ` Boris Ostrovsky
2017-03-31 19:50 ` [PATCH for 4.9 5/6] x86/emul: Drop swint_emulate infrastructure Andrew Cooper
2017-04-03  8:36   ` Paul Durrant
2017-04-03  9:38   ` Jan Beulich
2017-03-31 19:50 ` [PATCH for 4.9 6/6] x86/emul: Require callers to provide LMA in the emulation context Andrew Cooper
2017-04-03  8:40   ` Paul Durrant
2017-04-03 10:10   ` Jan Beulich
2017-04-05 16:24     ` Andrew Cooper
2017-04-06  6:58       ` Jan Beulich
2017-04-06  9:47         ` Andrew Cooper
2017-04-06 14:14           ` Jan Beulich
2017-04-06 16:32             ` Andrew Cooper
2017-04-07  8:35               ` Jan Beulich
2017-04-05 16:07   ` Jan Beulich

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