Re: [Xen-devel] [PATCH 3 of 7] Use a reserved pfn in the guest address space to store mem event rings

["Andres Lagar-Cavilla" <andres@xxxxxxxxxxxxxxxx>]

> On Thu, 2012-02-23 at 06:05 +0000, Andres Lagar-Cavilla wrote:
>
> Doers this mean that a guest can now potentially observe, or even modify
> it's own mem event ring? Are we sure there's no potential for havoc
> here?

This is the same mechanism we use to map the qemu ioreq and bufioreq
rings. These pfns lie within a reserved region as advertised by the e820
to the guest, so that's the protection we rely upon.

>
> Is there no scope for making these pages owned by the domain but not
> actually part of the P2M? We can cope with that for other types of magic
> page, can't we?

If you're referring to the qemu rings, I don't think so. They're like any
other page in the p2m. They get allocated with xc_populate_physmap_exact.

>
> I didn't atually dig into the implementation other than to see if it
> answered  my questions, although I did notice:
>
>> diff -r 99e6c9b9e971 -r 0e79f8005b6b tools/libxc/xc_hvm_build.c
>> --- a/tools/libxc/xc_hvm_build.c
>> +++ b/tools/libxc/xc_hvm_build.c
>> @@ -38,12 +38,15 @@
>>  #define SUPERPAGE_1GB_SHIFT   18
>>  #define SUPERPAGE_1GB_NR_PFNS (1UL << SUPERPAGE_1GB_SHIFT)
>>
>> -#define SPECIALPAGE_BUFIOREQ 0
>> -#define SPECIALPAGE_XENSTORE 1
>> -#define SPECIALPAGE_IOREQ    2
>> -#define SPECIALPAGE_IDENT_PT 3
>> -#define SPECIALPAGE_CONSOLE  4
>> -#define NR_SPECIAL_PAGES     5
>> +#define SPECIALPAGE_PAGING   0
>> +#define SPECIALPAGE_ACCESS   1
>> +#define SPECIALPAGE_SHARING  2
>> +#define SPECIALPAGE_BUFIOREQ 3
>> +#define SPECIALPAGE_XENSTORE 4
>> +#define SPECIALPAGE_IOREQ    5
>> +#define SPECIALPAGE_IDENT_PT 6
>> +#define SPECIALPAGE_CONSOLE  7
>> +#define NR_SPECIAL_PAGES     8
>
> Any reason to not simply append them?

Yes: I didn't want to change the existing magic pfn's. If I just append,
all pfns shift, and now the venerable store or ioreq pfn's are on
different locations.

Andres
>
> Ian.
>
>



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