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

[Wei Liu <wl@xen.org>]

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.

Wei.

> 
> Jan

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