Re: [PATCH v2] KVM: IOAPIC: only access APIC registers one dwordat a time

[Avi Kivity]

On 07/05/2010 06:47 AM, Xiao Guangrong wrote:
> Avi Kivity wrote:
>    
> > On 07/02/2010 11:00 AM, Xiao Guangrong wrote:
> >      
> > > The IOAPIC spec says:
> > >
> > > When accessing these registers, accesses must be done one dword at a
> > > time.
> > > For example, software should never access byte 2 from the Data
> > > register before
> > > accessing bytes 0 and 1. The hardware will not attempt to recover from
> > > a bad
> > > programming model in this case.
> > >
> > > So, this patch removes other width access
> > >
> > >
> > >        
> > The ioapic code also implements the ia64 iosapic.  I'm guessing that
> > does support 64-bit accesses.  Please check the iosapic documentation.
> >
> >      
> The iosapic also using 32-bit to access registers:
>
> All registers are accessed using 32-bit uncacheable loads and stores to a reserved memory location
> in system memory. This implies that to modify a field (e.g., a bit or a byte) in any register, the
> whole 32-bit register must be read, the field modified, and the 32 bits written back. Partial register
> access, or non-aligned register access, are implementation-defined by the I/O xAPIC and will not
> be compatible across different implementations. Also, registers that are described as 64 bits wide
> are accessed as multiple independent 32-bit registers.
>
> [ From<<  Intel Itanium Processor Family Interrupt Architecture Guide>>, P2-6 ]
>    

Ok.

> > There might be guests that use incorrect access despite the
> > documentation; if real hardware supports it, it should work.  So we need
> > to start with just a warning, and allow the access.  Later we can drop
> > the invalid access.
> >      
> If the OS contravene the spec, i thinks it's the OS's bug, also, i have tested some versions
> windows/linux guests, it's no broken, can we directly drop the other wide access?
>    

Well, there's the spec and there's real life, but in this case we can 
try and if we see a problem we'll re-add the other access length.


--
error compiling committee.c: too many arguments to function

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