RE: [PATCH v2 1/1] Fix PR16193 - gdbserver aborts.

["Tedeschi, Walfred" <walfred dot tedeschi at intel dot com>]

Hi Pedro,

In fact I didn't like this macro from the beginning on. And liked less when I saw it with AVX512. 
Therefore I took the time to rewrite it. It was too awful to read in the way it was. :)
:)

I will fix the rationale.

Thanks and regards,
-Fred

-----Original Message-----
From: Pedro Alves [mailto:palves@redhat.com] 
Sent: Monday, November 25, 2013 5:28 PM
To: Tedeschi, Walfred
Cc: mark.kettenis@xs4all.nl; gdb-patches@sourceware.org
Subject: Re: [PATCH v2 1/1] Fix PR16193 - gdbserver aborts.

On 11/25/2013 03:55 PM, Walfred Tedeschi wrote:
> The MPX patch has broken the I386_XSTATE_SIZE macro. For any xcr0 
> value entered return is 576. This patch fixes that and improves 
> readability of the macros.

Since you still didn't explain it, I took another closer look.
I can't believe how long it took me to spot it.  I kept staring at the BNDREGS and BNDCFG bits, but the issue is really in the AVX check.

>  /* Get I386 XSAVE extended state size.  */  #define 
> I386_XSTATE_SIZE(XCR0) \
> -    (((XCR0) & I386_XSTATE_BNDCFG) != 0 ? I386_XSTATE_BNDCFG_SIZE \
> -       : (((XCR0) & I386_XSTATE_BNDREGS) != 0 ? I386_XSTATE_BNDCFG_SIZE \
> -       : (((XCR0) & I386_XSTATE_AVX_SIZE) != 0 ? I386_XSTATE_AVX_SIZE \

The issue is here:

     : (((XCR0) & I386_XSTATE_AVX_SIZE) != 0 ? I386_XSTATE_AVX_SIZE \

That should have been:

     : (((XCR0) & I386_XSTATE_AVX) != 0 ? I386_XSTATE_AVX_SIZE \


Please fix the rationale in the commit log.  It's not true that I386_XSTATE_SIZE _always_ returns 576.  For MPX machines, the existing code returns the right value.  It's only non-MPX AVX machines that get the wrong value.  IOW, this one lines would be the equivalent smallest fix:

/* Get I386 XSAVE extended state size.  */ #define I386_XSTATE_SIZE(XCR0) \
    (((XCR0) & I386_XSTATE_BNDCFG) != 0 ? I386_XSTATE_BNDCFG_SIZE \
       : (((XCR0) & I386_XSTATE_BNDREGS) != 0 ? I386_XSTATE_BNDCFG_SIZE \
-       : (((XCR0) & I386_XSTATE_AVX_SIZE) != 0 ? I386_XSTATE_AVX_SIZE \
+       : (((XCR0) & I386_XSTATE_AVX) != 0 ? I386_XSTATE_AVX_SIZE \
       : I386_XSTATE_SSE_SIZE)))


Something like this:

"
The MPX patch has broken the I386_XSTATE_SIZE macro.  For AVX machines, it ends up returning I386_XSTATE_SSE_SIZE.  Where it first reads I386_XSTATE_AVX_SIZE, it should have read I386_XSTATE_AVX:

 #define I386_XSTATE_SIZE(XCR0) \
     (((XCR0) & I386_XSTATE_BNDCFG) != 0 ? I386_XSTATE_BNDCFG_SIZE \
        : (((XCR0) & I386_XSTATE_BNDREGS) != 0 ? I386_XSTATE_BNDCFG_SIZE \
 -       : (((XCR0) & I386_XSTATE_AVX_SIZE) != 0 ? I386_XSTATE_AVX_SIZE \
 +       : (((XCR0) & I386_XSTATE_AVX) != 0 ? I386_XSTATE_AVX_SIZE \
        : I386_XSTATE_SSE_SIZE)))

The patch goes a step further and improves readability of the macro, by adding a couple other auxiliary macros.

2013-11-25  Walfred Tedeschi  <walfred.tedeschi@intel.com>

	* i386-xstate.h (I386_XSTATE_MPX): New Macro.
	(I386_XSTATE_MPX_MASK): Makes use of I386_XSTATE_MPX.
	(HAS_MPX): New macro.
	(HAS_AVX): New macro.
	(I386_XSTATE_SIZE): Uses HAS_MPX and HAS_AVX.
"

> 2013-12-25  Walfred Tedeschi  <walfred.tedeschi@intel.com>

Longing for Christmas? ;-)


-> Oh! Yah! :)
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