Re: [Xen-devel] [PATCH v2 18/30] xen/x86: setup PVHv2 Dom0 ACPI tables

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 26.10.16 at 17:08, <roger.pau@xxxxxxxxxx> wrote:
> On Wed, Oct 26, 2016 at 08:10:50AM -0600, Jan Beulich wrote:
>> >>> On 26.10.16 at 13:35, <roger.pau@xxxxxxxxxx> wrote:
>> > On Wed, Oct 12, 2016 at 09:55:44AM -0600, Jan Beulich wrote:
>> >> Taking an abstract perspective I agree with Andrew that we should
>> >> be whitelisting here. However, as you already see from the list you
>> >> provided (which afaict is far from complete wrt ACPI 6.1), this may
>> >> become cumbersome already initially, not to speak of down the road.
>> > 
>> > I've initially used a back-listing approach. We can always change this 
>> > later 
>> > on.
>> > 
>> > So I've ended up crafting a new MADT, XSDT and RSDP. Note that I'm not 
>> > crafting a new custom RSDT (and in fact I'm setting rsdt_physical_address 
>> > = 
>> > 0 in the RSDP together with revision = 2). This is all placed in RAM 
>> > stolen 
>> > from the guest memory map and marked as E820_ACPI, which means that the 
>> > new 
>> > RSDP no longer resides below 1MB, and that the Dom0 kernel _must_ use the 
>> > rsdp_paddr provided in the start info, or else it's going to access the 
>> > native RSDP.
>> 
>> Hmm, for the RSDP I'm not sure. It might be better if we put it at the
>> same spot where the host one is, mapping a RAM page there with a
>> copy of the respective host page data. Otoh your approach allows
>> Dom0 to still find the real tables if need be, which has both up and
>> down sides.
> 
> The problem with putting it at the same page is that AFAICT there's a big 
> chance that other things (like EBDA or ROM) being at the same page, and 
> we would be shadowing them by mapping a RAM page over it, even if the 
> original data is copied. This hole area below 1MB is just a mess to deal 
> with TBH.

Unless it's page zero, what bad could such shadowing do?

Jan

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