xen-devel.lists.xenproject.org archive mirror
 help / color / mirror / Atom feed
From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: xen-devel <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH 2/2] x86/HVM: re-order operations in hvm_ud_intercept()
Date: Fri, 17 Jun 2016 10:37:30 +0100	[thread overview]
Message-ID: <0eed9bd9-acb9-0bb7-ae70-34aff1df9b28@citrix.com> (raw)
In-Reply-To: <5759A1D602000078000F3994@prv-mh.provo.novell.com>

On 09/06/16 16:05, Jan Beulich wrote:
>>>> On 09.06.16 at 16:27, <andrew.cooper3@citrix.com> wrote:
>> On 09/06/16 15:13, Jan Beulich wrote:
>>>>>> On 09.06.16 at 16:06, <andrew.cooper3@citrix.com> wrote:
>>>> On 09/06/16 13:31, Jan Beulich wrote:
>>>>>>>> On 09.06.16 at 13:34, <andrew.cooper3@citrix.com> wrote:
>>>>>> On 08/06/16 14:43, Jan Beulich wrote:
>>>>>>> Don't fetch CS explicitly, leverage the fact that hvm_emulate_prepare()
>>>>>>> already does (and that hvm_virtual_to_linear_addr() doesn't alter it).
>>>>>>>
>>>>>>> At once increase the length passed to hvm_virtual_to_linear_addr() by
>>>>>>> one: There definitely needs to be at least one more opcode byte, and we
>>>>>>> can avoid missing a wraparound case this way.
>>>>>>>
>>>>>>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>>>>>> I looked into this when you suggested it, but it latches the wrong eip
>>>>>> in the emulation state, and you will end up re-emulating the ud2a
>>>>>> instruction, rather than the following instruction.
>>>>> Where is there any latching of eip? All hvm_emulate_prepare() does
>>>>> is storing the regs pointer.
>>>> Oh - so it does.  I clearly looked over it too quickly.
>>>>
>>>> What wraparound issue are you referring to?  Adding 1 will cause
>>>> incorrect behaviour when the emulation prefix ends at the segment limit.
>>> I don't think so: The prefix together with the actual instruction
>>> encoding should be viewed as a single instruction, and it crossing
>>> the segment limit should #GP. It wrapping at the prefix/encoding
>>> boundary is the case that I'm specifically referring to (this case
>>> should also #GP, but wouldn't without this adjustment).
>> But the force emulation prefix specifically doesn't behave like other
>> prefixes.
>>
>> It doesn't count towards the 15 byte instruction limit, and if the
>> emulated instruction does fault, we want the fault pointing at the
>> emulated instruction, not the force prefix.  We should avoid making any
>> link.
> Well, are you saying placing such a prefix right below the boundary
> of a flat segment is _expected_ to get the instruction at address 0
> emulated? I don't think I could buy that. The patch makes no other
> connection between prefix and actual insn. And #GP because of
> such a boundary condition should imo point at the prefix; only all
> faults associated with the actual insn should point there.

Ok.  That sounds reasonable.  Would it be possible to add a small
comment to the code? Even with the commit message, I was confused as to
the nature of the +1.

Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
http://lists.xen.org/xen-devel

  parent reply	other threads:[~2016-06-17  9:37 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-06-08 13:35 [PATCH] x86/HVM: mis adjustments Jan Beulich
2016-06-08 13:42 ` [PATCH 1/2] x86/HVM: constify hvm_virtual_to_linear_addr()'s segment register parameter Jan Beulich
2016-06-09 11:25   ` Andrew Cooper
2016-06-08 13:43 ` [PATCH 2/2] x86/HVM: re-order operations in hvm_ud_intercept() Jan Beulich
2016-06-09 11:34   ` Andrew Cooper
2016-06-09 12:31     ` Jan Beulich
2016-06-09 14:06       ` Andrew Cooper
2016-06-09 14:13         ` Jan Beulich
2016-06-09 14:27           ` Andrew Cooper
2016-06-09 15:05             ` Jan Beulich
2016-06-17  8:19               ` Ping: " Jan Beulich
2016-06-17  9:37               ` Andrew Cooper [this message]
2016-06-17 10:01                 ` Jan Beulich

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=0eed9bd9-acb9-0bb7-ae70-34aff1df9b28@citrix.com \
    --to=andrew.cooper3@citrix.com \
    --cc=JBeulich@suse.com \
    --cc=xen-devel@lists.xenproject.org \
    --subject='Re: [PATCH 2/2] x86/HVM: re-order operations in hvm_ud_intercept()' \
    /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

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).