Re: [XEN v1 8/9] xen/arm: Other adaptations required to support 32bit paddr

[Julien Grall <julien@xxxxxxx>]

Hi,

On 20/12/2022 15:24, Ayan Kumar Halder wrote:
> On 16/12/2022 10:23, Julien Grall wrote:
> > Each adaptations are distinct, so I would prefer if they are in in 
> > separate patch.
> > This will also make clear which components you touched because I would 
> > be surprised if this is really the only place where we need 
> > adaptation. Maybe that's because you didn't compile everything (which 
> > is fine).
> > On 15/12/2022 19:32, Ayan Kumar Halder wrote:
> > > 1. Supersections are supported only for paddr greater than 32 bits.
> > Two questions:
> >  * Can you outline why we can't keep the code around?
> For PA_32, the following bitoperation will overflow :-
>
>              *ipa |= (paddr_t)(pte.supersec.extbase1) << 
> L1DESC_SUPERSECTION_EXT_BASE1_SHIFT;
>              *ipa |= (paddr_t)(pte.supersec.extbase2) << 
> L1DESC_SUPERSECTION_EXT_BASE2_SHIFT;
> Also, pte.supersec.extbase1 and pte.supersec.extbase2 are not valid for 
> PA_32. Refer xen/arch/arm/include/asm/short-desc.h
You are right about extbase1 and extbase2. See below for the 
supersection support.
> unsigned int extbase2:4;    /* Extended base address, PA[39:36] */
>
> unsigned int extbase1:4;    /* Extended base address, PA[35:32] */
>
> >  * Can you give a pointer to the Arm Arm that says supersection is not 
> > supported?
> I could not find any sentence in Arm Arm which says supersection is 
> **not** supported on 32 bit PA.
> However,
>
> Refer"ARM DDI 0487I.a ID081822", G5.4 "The VMSAv8-32 Short-descriptor 
> translation table format", G5-9163
> "Support for Supersections is **optional**, except that an 
> implementation that supports more than 32 bits of PA must also support 
> Supersections to provide access to the entire PA space."
> Also, G5.1.3 "Address spaces in VMSAv8-32", G5-9149
>
> "AArch32 defines two translation table formats. The Long-descriptor 
> format gives access to the full 40-bit IPA or PA space at a granularity 
> of 4KB. The Short-descriptor format:
>     Gives access to a 32-bit PA space at 4KB granularity.
>     Gives access to a 40-bit PA space, but only at 16MB granularity, by 
> the use of Supersections."
> from the above 2 snippets, I inferred that supersections are only 
> supported for PAs greater than 32 bits.
> I could not find any evidence of supersections supported for 32 bit PA.
From what you quoted above supersection is optional unless the 
processor support more than 32-bit PA. IOW, an implementer is free to 
implement the feature even if it is not strictly necessary when the 
processor only supports 32-bit PA. This because it is useful to reduce 
the TLB contention.
So if we want to #ifdef some code, then only the two lines using 
L1DESC_SUPERSECTION_EXT_BASE{1, 2}_SHIFT should be protected. The rest 
should stay as-is.
Cheers,

--
Julien Grall