Re: [Xen-devel] [PATCH v4 3/3] x86/ioreq server: Add HVMOP to map guest ram with p2m_ioreq_server to an ioreq server.

[Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>]

On 6/16/2016 6:02 PM, Jan Beulich wrote:
> > > > On 16.06.16 at 11:32, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
> > > On 6/14/2016 6:45 PM, Jan Beulich wrote:
> > > > > > > > On 19.05.16 at 11:05, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
> > > > > > @@ -914,6 +916,45 @@ int hvm_unmap_io_range_from_ioreq_server(struct domain
> > > > *d, ioservid_t id,
> > > > > >         return rc;
> > > > > >     }
> > > > > >     
> > > > > > +int hvm_map_mem_type_to_ioreq_server(struct domain *d, ioservid_t id,
> > > > > > +                                     uint16_t type, uint32_t flags)
> > > > > I see no reason why both can't be unsigned int.
> > > > Parameter type is passed in from the type field inside struct
> > > > xen_hvm_map_mem_type_to_ioreq_server,
> > > > which is a uint16_t, followed with a uint16_t pad. Now I am wondering,
> > > > may be we can just remove the pad
> > > > field in this structure and just define type as uint32_t.
> > > I think keeping the interface structure unchanged is the desirable
> > > route here. What I dislike is the passing around of non-natural
> > > width types, which is more expensive in terms of processing. I.e.
> > > as long as a fixed width type (which is necessary to be used in
> > > the public interface) fits in "unsigned int", that should be the
> > > respective internal type. Otherwise "unsigned long" etc.
> > >
> > > There are cases where even internally we indeed want to use
> > > fixed width types, and admittedly there are likely far more cases
> > > where internally fixed width types get used without good reason,
> > > but just like everywhere else - let's please not make this worse.
> > > IOW please use fixed width types only when you really need them.
> > OK. I can keep the interface, and using uint32_t type in the internal
> > routine
> > would means a implicit type conversion from uint16_6, which I do not think
> > is a problem.
> Just to reiterate: Unless there is a specific need, please avoid
> fixed width integer types for any internal use.
>
> > > > > > @@ -94,8 +96,16 @@ static unsigned long p2m_type_to_flags(p2m_type_t t, mfn_t 
> > > > > > mfn,
> > > > > >         default:
> > > > > >             return flags | _PAGE_NX_BIT;
> > > > > >         case p2m_grant_map_ro:
> > > > > > -    case p2m_ioreq_server:
> > > > > >             return flags | P2M_BASE_FLAGS | _PAGE_NX_BIT;
> > > > > > +    case p2m_ioreq_server:
> > > > > > +    {
> > > > > > +        flags |= P2M_BASE_FLAGS | _PAGE_RW;
> > > > > > +
> > > > > > +        if ( p2m->ioreq.flags & P2M_IOREQ_HANDLE_WRITE_ACCESS )
> > > > > > +            return flags & ~_PAGE_RW;
> > > > > > +        else
> > > > > > +            return flags;
> > > > > > +    }
> > > > > Same here (for the missing _PAGE_NX) plus no need for braces.
> > > > I'll remove the brace. And we do not need to set the _PAGE_NX_BIT, like
> > > > the p2m_ram_ro case I guess.
> > > I hope you mean the inverse: You should set _PAGE_NX_BIT here.
> > Oh, right. I meant the reverse. Thanks for the remind. :)
> > And I have a question,  here in p2m_type_to_flags(), I saw current code
> > uses _PAGE_NX_BIT
> > to disable the executable permission,  and I wonder, why don't we choose
> > the _PAGE_NX,
> > which is defined as:
> >
> > #define _PAGE_NX       (cpu_has_nx ? _PAGE_NX_BIT : 0)
> >
> > How do we know for sure that bit 63 from pte is not a reserved one
> > without checking
> > the cpu capability(the cpu_has_nx)? Is there any other reasons, i.e. the
> > page tables might
> > be shared with IOMMU?
> Please wait for Andrew to confirm this (or correct me) - there are
> some differences between AMD and Intel, and iirc the bit gets
> ignored by AMD when NX is off.
Hi Andrew, sorry to bother you. Any comments on this? Thanks! :)

Yu

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