Re: [Qemu-devel] [PATCH 04/11] target/arm: Record the GP bit for a page in MemTxAttrs

[Peter Maydell]

On Mon, 28 Jan 2019 at 21:08, Richard Henderson
<address@hidden> wrote:
>
> On 1/22/19 5:26 AM, Peter Maydell wrote:
> >> +        guarded |= extract64(descriptor, 50, 1);  /* GP */
> >
> > Do we need to do the logical-OR here? Since this is a
> > block/page entry bit with no similar bit in the table
> > descriptors, there's no merging to be done (ie we
> > only execute this code once and 'guarded' will always
> > be 'false' before execution of the |=.)
>
> The document that I have has exactly one sentence about this, and does not
> specify whether the bit is akin to the page table attributes (which appear at
> every table level) or not.

Translation table descriptor formats come in four flavours:
 * Invalid
 * Table (which gives the address of the next level table)
 * Block (which gives the address of a large lump of memory)
 * Page (which gives the address of a page)

The GP bit documented to be in Block and Page entries, not
Table (which is how you've coded it).

> As written above, this will execute more than once.

I don't see how -- all the code paths forward from
"guarded |= extract64(descriptor, 50, 1);" reach a
"break" statement that terminates the loop, don't they?

thanks
-- PMM