On Sun, 2010-10-31 at 16:28 +0000, Jeremy Fitzhardinge wrote:
> On 10/31/2010 02:13 AM, Ian Campbell wrote:
> >> The 3rd is certainly simplest, at the cost of wasting a trivial amount
> >> of memory.
> > Doesn't Linux avoid using the lowest 1M anyway? (obviously apart from
> > the start of day probing for firmware tables etc).
>
> No, it tries to use most of it I think. It will tend to avoid the low
> 64k (maybe more) to avoid BIOS bugs.
It'll be interesting to see what effect Vista's avoidance of the whole
region (so I hear) has on BIOS vendors... (I think we can all guess)
> >> Unfortunately it crashes early. Sigh, will try and sort it
> >> out this afternoon.
> > Strange!
>
> I didn't get a chance to poke at it again, but in retrospect, I think
> there are various "must succeed" allocations in low memory. We don't
> need those allocations (things like AP boot trampoline, etc), but we
> don't bother to stub them out or prevent them from happening. Reducing
> the system to one with *no* allocatable memory below 1M is just too
> strange, and would be a continuous source of problems in the future.
Agreed, we should try and mimic native as far as possible in this regard
or I fear we will see a never ending stream of little quirks and
oddities related to this sort of thing.
> Of the other two options, I think your original approach is going to be
> simplest. E820 gap filling wouldn't be too bad, but we'd end up having
> to add a bit of gap-tracking logic to the E820 loop which isn't
> currently there.
It would also make us susceptible to perhaps being a bit fragile in the
face of unexpectedly insane e820s coming from the BIOS.
> Ignoring sub-1M gaps is simpler (and it needn't be
> conditional on xen_initial_domain(), because we would never expect to
> see anything strange sub-1M in a domU, and if there is, we should still
> be careful of it in case something odd is going on).
Absolutely.
I wonder if we shouldn't also do the following (note: untested). Since
Xen avoids using the sub-1M region for anything I think it is reasonable
to give the whole lot over to domain 0 for the purposes of finding
firmware table stashed in odd locations etc.
Ian.
diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index ebb74ec..ab086e5 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -2321,7 +2321,7 @@ __init void xen_ident_map_ISA(void)
xen_raw_printk("Xen: setup ISA identity maps\n");
- for (pa = ISA_START_ADDRESS; pa < ISA_END_ADDRESS; pa += PAGE_SIZE) {
+ for (pa = 0; pa < ISA_END_ADDRESS; pa += PAGE_SIZE) {
pte_t pte = mfn_pte(PFN_DOWN(pa), PAGE_KERNEL_IO);
if (HYPERVISOR_update_va_mapping(PAGE_OFFSET + pa, pte, 0))