Re: [Xen-devel] [RFC] DOMCTL_memattrs_op : a new DOMCTL to play with stage-2 page attributes

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.07.17 at 20:35, <sstabellini@xxxxxxxxxx> wrote:
> On Tue, 4 Jul 2017, Jan Beulich wrote:
>> >>> On 03.07.17 at 19:58, <sstabellini@xxxxxxxxxx> wrote:
>> > On Mon, 3 Jul 2017, Zhongze Liu wrote:
>> >> 2017-07-03 22:25 GMT+08:00 Jan Beulich <JBeulich@xxxxxxxx>:
>> >> >>>> On 30.06.17 at 22:15, <blackskygg@xxxxxxxxx> wrote:
>> >> >> /* flags for XEN_DOMCTL_MEMATTRS_OP_SET_PERMISSIONS */
>> >> >> #define XEN_DOMCTL_MEMATTRS_ACCESS_N         0x00U
>> >> >> #define XEN_DOMCTL_MEMATTRS_ACCESS_R         (0x01U<<0)
>> >> >> #define XEN_DOMCTL_MEMATTRS_ACCESS_W         (0x01U<<1)
>> >> >> #define XEN_DOMCTL_MEMATTRS_ACCESS_X         (0x01U<<2)
>> >> >> #define XEN_DOMCTL_MEMATTRS_ACCESS_RW        \
>> >> >> (XEN_DOMCTL_MEMATTRS_ACCESS_R|XEN_DOMCTL_MEMATTRS_ACCESS_W)
>> >> >> #define XEN_DOMCTL_MEMATTRS_ACCESS_RX        \
>> >> >> (XEN_DOMCTL_MEMATTRS_ACCESS_R|XEN_DOMCTL_MEMATTRS_ACCESS_X)
>> >> >> #define XEN_DOMCTL_MEMATTRS_ACCESS_WX        \
>> >> >> (XEN_DOMCTL_MEMATTRS_ACCESS_W|XEN_DOMCTL_MEMATTRS_ACCESS_X)
>> >> >> #define XEN_DOMCTL_MEMATTRS_ACCESS_RWX        \
>> >> >> (XEN_DOMCTL_MEMATTRS_ACCESS_RW|XEN_DOMCTL_MEMATTRS_ACCESS_X)
>> >> >
>> >> > ... with this basically duplicating
>> >> > XENMEM_access_op_{set,get}_access I now wonder whether
>> >> > we don't already have all you need (apart from an ARM variant of
>> >> > DOMCTL_pin_mem_cacheattr).
>> >> 
>> >> In fact, there isn't much description on the usage of this
>> >> interface, so I turned to the implementation in
>> >> xen/common/mem_access.c, where I see this
>> >> interface invoking  p2m_set_mem_acess, which further invokes
>> >> set_mem_acess and finally
>> >> p2m->set_entry(), so I guess this might be the right interface to use.
>> >> To confirm the guess, I turned to Stabellini for help, and he told me
>> >> that XENMEM_access_op
>> >> is "for getting very detail info on what the guest is accessing", and
>> >> might not be suitable
>> >> for this scenario, so I just gave up using it, and that's why I have this 
> RFC.
>> >> I'll re-confirm this with Stabellini.
>> > 
>> > I thought that those two hypercalls were meant to be used for mem_access
>> > and vm_event operations, as in xen/arch/arm/mem_access.c and
>> > xen/arch/x86/mm/mem_access.c. The only caller is
>> > tools/tests/xen-access/xen-access.c. They are enabled separatly as part
>> > of the mem_access interface: their build is conditional to
>> > CONFIG_HAS_MEM_ACCESS. Unless we want to move them from XENMEM_access_*
>> > to DOMCTL_* operations, I don't think they could be used?
>> 
>> For one, we could remove the CONFIG_HAS_MEM_ACCESS around
>> them if a broader use is planned. And in general we should try to
>> avoid having two ways of doing the same thing, unless backwards
>> compatibility makes this a requirement. Hence if a new, better way
>> is to be introduced, the old one should at once go away. Finally, I'm
>> still unconvinced a new DOMCTL_* is better here than a (tool stack
>> only) XENMEM_*, but I agree the boundary between when to use
>> what is at best fuzzy.
> 
> Do we maintain ABI compatibility for XENMEM_* hypercalls? I think we do,
> don't we? Also, XENMEM_* hypercalls are usually available to both
> guests and toolstack, right?

There are ones of both kinds. Which one belongs to which group
depends on whether they sit in a __XEN__/__XEN_TOOLS__ framed
section.

Jan


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