Re: [Xen-devel] [PATCH v6 05/11] x86/hyperv: setup hypercall page

[Jan Beulich <jbeulich@xxxxxxxx>]

On 03.02.2020 16:21, Roger Pau Monné wrote:
> On Mon, Feb 03, 2020 at 03:07:24PM +0000, Wei Liu wrote:
>> On Mon, Feb 03, 2020 at 04:01:54PM +0100, Roger Pau Monné wrote:
>>> On Fri, Jan 31, 2020 at 05:49:24PM +0000, Wei Liu wrote:
>>>> Hyper-V uses a technique called overlay page for its hypercall page. It
>>>> will insert a backing page to the guest when the hypercall functionality
>>>> is enabled. That means we can use a page that is not backed by real
>>>> memory for hypercall page.
>>>>
>>>> Use the top-most addressable page for that purpose. Adjust e820 map
>>>> accordingly.
>>>
>>> Can you add this is done to avoid page shattering and to make sure
>>> Xen isn't overwriting any MMIO area which might be present at lower
>>> addresses?
>>
>> NP.
>>
>>>
>>>> +
>>>> +static void __init e820_fixup(struct e820map *e820)
>>>> +{
>>>> +    uint64_t s = HV_HCALL_MFN << PAGE_SHIFT;
>>>> +
>>>> +    if ( !e820_add_range(e820, s, s + PAGE_SIZE, E820_RESERVED) )
>>>
>>> I think end should be s + PAGE_SIZE - 1, or else it expands across two
>>> pages?
>>
>> No, it shouldn't.
>>
>> E820 entry records the size of the region, which is calculated as
>> end-start. The one usage in pv/shim.c follows the same pattern here.
> 
> Hm, I see. I'm not sure this is correct, I think the e820 entry
> should look like:
> 
> addr = s;
> size = PAGE_SIZE - 1;
> 
> As ranges on the e820 are inclusive, so if size ends up being
> PAGE_SIZE then the entry would expand across two pages.

Ranges can sensibly be inclusive only when specified as [start,end]
tuples. (start,size) pairs make no sense for representing
[start,start+size], they only make sense for [start,start+size).
Otherwise, as in your example above, size taken on its own is off
by one (i.e. is rather "last byte" than "size").

Modern Linux, when logging the memory map, indeed subtracts 1 from
the sum of addr and size, to show an inclusive range.

Jan

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