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Date: Mon, 05 Dec 2011 16:14:20 +0200
From: Avi Kivity <avi@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] exec.c: Allow memory region start_addr and
 region_offset to vary in low bits
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: Paul Brook <paul@codesourcery.com>, qemu-devel@nongnu.org, patches@linaro.org

On 12/05/2011 04:01 PM, Peter Maydell wrote:
> On 5 December 2011 13:40, Avi Kivity <avi@redhat.com> wrote:
> > On 12/05/2011 01:01 PM, Peter Maydell wrote:
> >> @@ -2677,7 +2674,11 @@ void cpu_register_physical_memory_log(target_phys_addr_t start_addr,
> >>      if (phys_offset == IO_MEM_UNASSIGNED) {
> >>          region_offset = start_addr;
> >>      }
> >> -    region_offset &= TARGET_PAGE_MASK;
> >> +    /* Adjust the region offset to account for the start_addr possibly
> >> +     * not being page aligned, so we end up passing the IO functions
> >> +     * the true offset from the start of the region.
> >> +     */
> >> +    region_offset -= (start_addr & ~TARGET_PAGE_MASK);
> >>      size = (size + TARGET_PAGE_SIZE - 1) & TARGET_PAGE_MASK;
> >>      end_addr = start_addr + (target_phys_addr_t)size;
> >>
> >
> > region_offset is added to iotlb in tlb_set_page(), smashing the low bits
> > with your change.  It's safe in subpage, since that doesn't happen there.
>
> OK, but we only need to avoid trashing the bottom 5 bits, right?

All TARGET_PAGE_BITS of them.

> So we could do
>     region_offset -= (start_addr & ~TARGET_PAGE_MASK);
>     if (size >= TARGET_PAGE_SIZE) {
>         region_offset &= ~0x1F; /* can make this a #define IO_MEM_MASK */
>     }
>
> which would allow regions to start on 0x20 granularity, or byte granularity
> if they're less than a page in size (and so guaranteed to be subpages only).
>

An alternative is to stash region_offset somewhere else.  There's
CPUTLBEntry::addend, see comment above its definition.

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