Re: [PATCH 2/4] target/arm: Fixup contiguous first-fault and no-fault loads

[LIU Zhiwei]

On 2020/12/9 4:16, Richard Henderson wrote:

> On 12/6/20 10:46 PM, LIU Zhiwei wrote:
> > First-fault or no-fault doesn't mean only access one page.
> >       
> But the implementation is *allowed* to access only one page.
> Thus the comment:
>
> > -    /*
> > -     * MemSingleNF is allowed to fail for any reason.  We have special
> > -     * code above to handle the first element crossing a page boundary.
> > -     * As an implementation choice, decline to handle a cross-page element
> > -     * in any other position.
> > -     */
> >       
>     
    From the pseudo code,  I see  that we should handle the first
    element in first-fault follow Mem. And the other elements in this
    function should follow  MemNF.
    
    I have some questions here：
    
    1. Why  do special process to the first element if it crosses pages
    in no-fault? Because it's not aligned?
    
// MemNF[] - non-assignment form
// =============================
(bits(8*size), boolean) MemNF[bits(64) address, integer size, AccType acctype]
> assert size IN {1, 2, 4, 8, 16};
> bits(8*size) value;
> aligned = (address == Align(address, size));
> A = SCTLR[].A;
> if !aligned && (A == '1') then
>     return (bits(8*size) UNKNOWN, TRUE);
> atomic = aligned || size == 1;
> if !atomic then
>     (value<7:0>, bad) = MemSingleNF[address, 1, acctype, aligned];
>     if bad then
>         return (bits(8*size) UNKNOWN, TRUE);
>     // For subsequent bytes it is CONSTRAINED UNPREDICTABLE whether an unaligned Device memory
>     // access will generate an Alignment Fault, as to get this far means the first byte did
>     // not, so we must be changing to a new translation page.
>     if !aligned then
>            c = ConstrainUnpredictable(Unpredictable_DEVPAGE2);
>     assert c IN {Constraint_FAULT, Constraint_NONE};
>     if c == Constraint_NONE then aligned = TRUE;
>     for i = 1 to size-1
>         (value<8*i+7:8*i>, bad) = MemSingleNF[address+i, 1, acctype, aligned];
>     if bad then
>         return (bits(8*size) UNKNOWN, TRUE);
> else
>     (value, bad) = MemSingleNF[address, size, acctype, aligned];
>     if bad then
>         return (bits(8*size) UNKNOWN, TRUE);
> return (value, FALSE);
>     
    
    
    2.  Why it doesn't access the second page like the first page? I
    think they should obey the same MemSingleNF implementation.
    
> r~
>