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Re: [Qemu-devel] [PATCH] i386/translate: ignore 0x67 (PREFIX_ADR) on TAR
From: |
Laszlo Ersek |
Subject: |
Re: [Qemu-devel] [PATCH] i386/translate: ignore 0x67 (PREFIX_ADR) on TARGET_X86_64 && CODE64() |
Date: |
Mon, 27 May 2013 01:45:18 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130513 Thunderbird/17.0.6 |
On 05/26/13 10:33, Paolo Bonzini wrote:
> Il 26/05/2013 01:23, Richard Henderson ha scritto:
>> On 2013-05-24 14:37, Laszlo Ersek wrote:
>>> @@ -4813,7 +4813,11 @@ static target_ulong disas_insn(CPUX86State
>>> *env, DisasContext *s,
>>> /* 0x66 is ignored if rex.w is set */
>>> dflag = 2;
>>> }
>>> - if (!(prefixes & PREFIX_ADR)) {
>>> + if (prefixes & PREFIX_ADR) {
>>> + /* flip it back, 0x67 should have no effect */
>>> + aflag ^= 1;
>>> + }
>>> + else {
>>> aflag = 2;
>>> }
>>> }
>>>
>>
>> Agreed that there's a bug here. I'm thinking it would be clearer to not
>> write this as yet another flip, but understand that unlike dflag, aflag
>> can only be either 1 or 2 in 64-bit mode.
>>
>> I'm thinking of something more like this:
>>
>> diff --git a/target-i386/translate.c b/target-i386/translate.c
>> index 0aeccdb..bf772aa 100644
>> --- a/target-i386/translate.c
>> +++ b/target-i386/translate.c
>> @@ -4813,9 +4813,8 @@ static target_ulong disas_insn(CPUX86State *env,
>> DisasContext *s,
>> /* 0x66 is ignored if rex.w is set */
>> dflag = 2;
>> }
>> - if (!(prefixes & PREFIX_ADR)) {
>> - aflag = 2;
>> - }
>> + /* 0x67 toggles between 64-bit and 32-bit addressing. */
>> + aflag = (prefixes & PREFIX_ADR ? 1 : 2);
>
> Isn't that just "aflag++"? Needs a comment of course ("toggle between
> 32- and 64-bit, not 16- and 32-bit.").
I finally looked up in the SDM what the 0x67 (address-size override)
prefix does. Apparently,
2.1.1 Instruction Prefixes
[...] The address-size override prefix (67H) allows programs to
switch between 16- and 32-bit addressing. Either size can be the
default; the prefix selects the non-default size. [...]
CMPS/CMPSB/CMPSW/CMPSD/CMPSQ -- Compare String Operands
[...] In 64-bit mode, the instruction's default address size is 64
bits, 32 bit address size is supported using the prefix 67H. [...]
Assuming that
- aflag==0 means 16-bit address,
- aflag==1 means 32-bit address,
- aflag==2 means 64-bit address,
- and bit0 in "s->code32" carries "aflag" corresponding to the default
address size in 32-bit mode,
I think Richard's patch is correct (my approach was only to restore the
pre-patch logic without having any clue about the variables' contents).
I believe aflag++ is incorrect if the current default address size for
32-bit is 16-bit (ie. (s->code32 & 1) == 0). In this case the first XOR
(seeing the 0x67 prefix) flips it to 1, and the increment would change
it to 2. aflag==2 corresponds to 64-bit address, but in 64-bit mode with
the 0x67 prefix we must choose 32-bit.
(IOW in 32-bit mode the meaning of the 0x67 prefix is not absolute but
relative.)
Laszlo