Re: [Xen-devel] [PATCH v3 3/5] x86/hyperv: provide percpu hypercall input page

[Michael Kelley <mikelley@microsoft.com>]

From: Wei Liu <wl@xen.org> Sent: Tuesday, January 7, 2020 8:34 AM
> 
> On Mon, Jan 06, 2020 at 11:27:18AM +0100, Jan Beulich wrote:
> > On 05.01.2020 17:47, Wei Liu wrote:
> > > Hyper-V's input / output argument must be 8 bytes aligned an not cross
> > > page boundary. The easiest way to satisfy those requirements is to use
> > > percpu page.
> >
> > I'm not sure "easiest" is really true here. Others could consider adding
> > __aligned() attributes as easy or even easier (by being even more
> > transparent to use sites). Could we settle on "One way ..."?
> 
> Do you mean something like
> 
>    struct foo __aligned(8);
> 
>    hv_do_hypercall(OP, virt_to_maddr(&foo), ...);
> 
> ?
> 
> I don't think this is transparent to user sites. Plus, foo is on stack
> which is 1) difficult to get its maddr, 2) may cross page boundary.
> 
> If I misunderstood what you meant, please give me an example here.
> 
> >
> > > @@ -83,14 +84,33 @@ static void __init setup_hypercall_page(void)
> > >      wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> > >  }
> > >
> > > +static void setup_hypercall_pcpu_arg(void)
> > > +{
> > > +    void *mapping;
> > > +
> > > +    mapping = alloc_xenheap_page();
> > > +    if ( !mapping )
> > > +        panic("Failed to allocate hypercall input page for %u\n",
> >
> > "... for CPU%u\n" please.
> >
> > > +              smp_processor_id());
> > > +
> > > +    this_cpu(hv_pcpu_input_arg) = mapping;
> >
> > When offlining and then re-onlining a CPU, the prior page will be
> > leaked.
> 
> Right. Thanks for catching this one.
> 
> >
> > > --- a/xen/include/asm-x86/guest/hyperv.h
> > > +++ b/xen/include/asm-x86/guest/hyperv.h
> > > @@ -51,6 +51,8 @@ static inline uint64_t hv_scale_tsc(uint64_t tsc, uint64_t scale,
> > >
> > >  #ifdef CONFIG_HYPERV_GUEST
> > >
> > > +#include <xen/percpu.h>
> > > +
> > >  #include <asm/guest/hypervisor.h>
> > >
> > >  struct ms_hyperv_info {
> > > @@ -63,6 +65,8 @@ struct ms_hyperv_info {
> > >  };
> > >  extern struct ms_hyperv_info ms_hyperv;
> > >
> > > +DECLARE_PER_CPU(void *, hv_pcpu_input_arg);
> >
> > Will this really be needed outside of the file that defines it?
> >
> 
> This can live in a private header for the time being.
> 
> > Also, while looking at this I notice that - despite my earlier
> > comment when giving the respective, sort-of-conditional ack -
> > there are (still) many apparently pointless __packed attributes
> > in hyperv-tlfs.h. Care to comment on this?
> 
> Again, that's a straight import from Linux. I tried not to deviate too
> much. A commit in Linux (ec084491727b0) claims "compiler can add
> alignment padding to structures or reorder struct members for
> randomization and optimization".
> 
> I just checked all the packed structures. They seem to have all the
> required manual paddings already. I can only assume they tried to erred
> on the safe side.

Correct.  The __packed attribute was added only about a year ago
after somebody on LKML noticed that the structures were not packed.
Some discussion ensued, but the consensus was to add __packed due
to general  paranoia about what the compiler might do even though
individual fields are aligned to their natural boundary.

Michael

> 
> Wei.
> 
> >
> > Jan

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