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Re: [Xen-devel] [PATCH v2] x86/mm/p2m: don't needlessly limit MMIO mapping order to 4k
- To: Jan Beulich <JBeulich@xxxxxxxx>, Paul Durrant <paul.durrant@xxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 25 Oct 2018 15:36:37 +0100
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- Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>
- Delivery-date: Thu, 25 Oct 2018 14:36:57 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 25/10/18 15:28, Jan Beulich wrote:
>>>> On 17.10.18 at 16:24, <paul.durrant@xxxxxxxxxx> wrote:
>> --- a/xen/arch/x86/mm/p2m.c
>> +++ b/xen/arch/x86/mm/p2m.c
>> @@ -2081,14 +2081,11 @@ static unsigned int mmio_order(const struct domain
>> *d,
>> unsigned long start_fn, unsigned long nr)
>> {
>> /*
>> - * Note that the !iommu_use_hap_pt() here has three effects:
>> - * - cover iommu_{,un}map_page() not having an "order" input yet,
>> - * - exclude shadow mode (which doesn't support large MMIO mappings),
>> - * - exclude PV guests, should execution reach this code for such.
>> - * So be careful when altering this.
>> + * PV guests or shadow-mode HVM guests must be restricted to 4k
>> + * mappings.
> Since you've already posted a patch to add order parameters to
> IOMMU map/unmap, I'd prefer the respective part of the comment
> to go away only when the order value really can be passed through.
> This then requires permitting non-zero values only when the IOMMUs
> also allow for the respective page sizes.
>
> I am in particular not convinced that the time needed to carry out
> the hypercall is going to be low enough even for 512 4k pages: You
> need to take into account flushes, including those potentially
> needed for ATS. You don't provide any proof that flushes are
> always delayed and batched, nor do I think this is uniformly the
> case.
I haven't had time to pick this back up since v1.
The long and the short of it is that we allow order 1G loops for regular
RAM, even in !shared_pt mode.
From an "interaction with the IOMMU" point of view, mappings over
regular RAM are no different to mappings over MMIO, so they should
behave consistently.
~Andrew
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