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Re: [Qemu-devel] sub-page-sized mmio regions and address passed to read/

From: Avi Kivity
Subject: Re: [Qemu-devel] sub-page-sized mmio regions and address passed to read/write fns
Date: Mon, 05 Dec 2011 11:26:40 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:8.0) Gecko/20111115 Thunderbird/8.0

On 12/04/2011 11:15 PM, Peter Maydell wrote:
> On 4 December 2011 12:17, Avi Kivity <address@hidden> wrote:
> > On 12/02/2011 04:49 PM, Peter Maydell wrote:
> >> However what I found is that the addresses passed to the read/write
> >> functions aren't what I would expect. For instance if the board
> >> maps the container at address 0x1e000000, then a read from 0x1e000100
> >> goes to the functions given by a9_gic_cpu_ops, as it should. However,
> >> the offset parameter that the read function is passed is not 0x0
> >> (offset from the start of the a9mp-gic-cpu region) but 0x100 (offset
> >> from the start of the page, I think).
> >>
> >> Is this expected behaviour? I certainly wasn't expecting it...
> >
> > A while ago this was the behaviour across the board.  Then 8da3ff1809747
> > changed addresses to be relative, but apparently missed the subpage case.
> Having looked a bit more closely at the code I think this is what
> the comment at the top of cpu_register_physical_memory_log() is
> referring to:
> # Both start_addr and region_offset are rounded down to a page boundary
> # before calculating this offset.  This should not be a problem unless
> # the low bits of start_addr and region_offset differ.
> In the case of a subregion at a non-page-aligned-address the
> start_addr is not page aligned, but the region_offset is zero,
> in the usual case, so we have differing low bits.

Not an issue in the subpage code.  As long as you extract the mmio index
before adding region_offset, you're fine (as the mmio index resides in
the low order bits).

> >> I looked through the code that's getting called for reads, and
> >> it looks to me like exec.c:subpage_readlen() is causing this.
> >> We look up the subpage_t based on the address within the page,
> >> but we don't then adjust the address we pass to io_mem_read
> >> (except by region_offset, which I take from the comment at the
> >> top of cpu_register_physical_memory_log() to be for something
> >> else.)
> > I think you can use subpage_t's region_offset array for this (adding
> > into it, of course, so the original value remains).
> Yes. I think the correction has to be calculated and applied in
> cpu_register_physical_memory_log() -- for a region which starts
> at a non-page-aligned address and extends over more than a page
> the correcting offset needs to be applied for the whole region,
> not just the first partial page.

In that case we have to use subpages for full pages.  But better to just
assert() that this never happens for now.

error compiling committee.c: too many arguments to function

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