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finding all the places in an AddressSpace that alias a specific address
From: |
Peter Maydell |
Subject: |
finding all the places in an AddressSpace that alias a specific address |
Date: |
Fri, 12 Mar 2021 16:05:58 +0000 |
I'm trying to fix an issue in Arm M-profile, where on reset the
CPU is supposed to load its initial PC and SP from a vector table.
This goes wrong if the vector table is in a guest image file (loaded
by the rom-blob loader) at address X which is not the same as the vector
table address Y but which is an aliased view of the same underlying RAM
(ie accesses to both X and Y go to the same real guest RAM).
Really the problem here is a reset-ordering one: the CPU reset code
runs before the hw/core/loader.c code has written the guest image
files to RAM. We currently try to work around that with:
rom = rom_ptr(vecbase, 8);
if (rom) {
/* Address zero is covered by ROM which hasn't yet been
* copied into physical memory.
*/
initial_msp = ldl_p(rom);
initial_pc = ldl_p(rom + 4);
} else {
/* Address zero not covered by a ROM blob, or the ROM blob
* is in non-modifiable memory and this is a second reset after
* it got copied into memory. In the latter case, rom_ptr
* will return a NULL pointer and we should use ldl_phys instead.
*/
initial_msp = ldl_phys(s->as, vecbase);
initial_pc = ldl_phys(s->as, vecbase + 4);
}
But the rom_ptr() check only matches if the ROM blob
is at the actual same guest address as the vector table base.
Arguably what we should try to sort out is the reset-ordering problem,
but that's a big job. I thought maybe we could for the moment at least
make the workaround we have today handle aliases.
For that I would need to find all the addresses in an AddressSpace
that alias vecbase, so that we can call rom_ptr() on all of them.
I think something like this would work:
hwaddr xlat, len;
RCU_READ_LOCK_GUARD();
FlatView *fv = address_space_to_flatview(s->as);
MemoryRegion *main_mr = flatview_translate(fv, vecbase, &xlat, &len,
false, MEMTXATTRS_UNSPECIFIED);
flatview_for_each_range(fv, callback_fn, ...);
where callback_fn() does:
if (mr != main_mr) {
return;
}
do arithmetic with flatview range start and len, etc to figure
out the overall address in the AS corresponding to vecbase
rom = rom_ptr(that_addr);
...
Does this seem like it would work ? Is there a nicer way ?
thanks
-- PMM
- finding all the places in an AddressSpace that alias a specific address,
Peter Maydell <=