Re: [PATCH] [RFC] More fp instructions for realmode emulation (Enables booting OS/2 as a HVM guest on Intel/VT hardware)

[Keir Fraser <keir.fraser@eu.citrix.com>]

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I¹ll have some time to do it this afternoon.

 -- Keir

On 3/3/08 12:18, "Trolle Selander" <trolle.selander@gmail.com> wrote:

> That's what I was worried about. I honestly had zilch experience working with
> assembler-level x87 stuff before Friday afternoon when I whipped up this
> patch, mainly to see if the fp instructions were all that was missing to get
> OS/2 on VT to work. I'll see if I can find time to make an exception-safe
> version of this sometime this week. I'm not worrying about life being too
> short, but rather that March 2008 is too dense... ;)
> 
> On Sat, Mar 1, 2008 at 10:18 AM, Keir Fraser <keir.fraser@eu.citrix.com>
> wrote:
>> Intel SDM Volume 1 Section 8.6 ("X87 FPU Exception Synchronization") is very
>> helpful in this regard. It confirms my suspicions, that we will need to wrap
>> the FPU inline asms in some fixup glue. The only exceptions to this rule are
>> the FN* (no-wait) instructions. Properly speaking we should also check for
>> unmasked exceptions becoming pending after any FPU instruction and set the
>> faulting CS:IP in the FPU context block appropriately. But life is perhaps
>> too short. :-)
>> 
>>  -- Keir
>> 
>> 
>> On 1/3/08 08:46, "Keir Fraser" <keir.fraser@eu.citrix.com> wrote:
>> 
>>> Regarding point 1: Don't FPU instructions that raise an exception only log
>>> that in the status word, to be picked up by the next FPU instruction which
>>> checks for exceptions? It strikes me that we can have previously loaded up a
>>> status word that requires an exception to occur on FWAIT, in which case
>>> semantics are broken. More worryingly, if any of the instructions you have
>>> added can cause an exception, we will crash Xen because you have not added
>>> fixup handling. Unfortunately I'm not really up on exactly how FPU exception
>>> handling works: are you?
>> 
> 
> 



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