Re: [Xen-devel] [PATCH v4 5/7] x86/hyperv: provide percpu hypercall input page

[Wei Liu <wl@xen.org>]

On Thu, Jan 23, 2020 at 04:45:38PM +0100, Jan Beulich wrote:
> On 22.01.2020 21:23, Wei Liu wrote:
> > --- a/xen/arch/x86/guest/hyperv/hyperv.c
> > +++ b/xen/arch/x86/guest/hyperv/hyperv.c
> > @@ -27,7 +27,10 @@
> >  #include <asm/guest/hyperv-tlfs.h>
> >  #include <asm/processor.h>
> >  
> > +#include "private.h"
> > +
> >  struct ms_hyperv_info __read_mostly ms_hyperv;
> > +DEFINE_PER_CPU_READ_MOSTLY(void *, hv_pcpu_input_arg);
> 
> Would it perhaps be helpful to make recognizable that this can hold
> up to a page's worth of data, either by its type or by a slightly
> different name?

I will change this to hv_pcpu_input_arg_page instead.

> 
> > @@ -119,14 +122,36 @@ static void __init setup_hypercall_page(void)
> >      }
> >  }
> >  
> > +static void setup_hypercall_pcpu_arg(void)
> > +{
> > +    void *mapping;
> > +
> > +    if ( this_cpu(hv_pcpu_input_arg) )
> > +        return;
> > +
> > +    mapping = alloc_xenheap_page();
> > +    if ( !mapping )
> > +        panic("Failed to allocate hypercall input page for CPU%u\n",
> > +              smp_processor_id());
> 
> panic() is likely fine for the BSP, but I don't think any AP should
> be able to bring down the system because of memory shortage. Such
> an AP should simply not come online. (Even for the BSP the question
> is whether Xen would be able to run despite failure here.)

This is no different than what is already done for Xen on Xen, i.e.
failure in setting up AP for any reason is fatal.

start_secondary doesn't even handling any failure by itself or
propagate failure back to caller.

Rewinding is a bit complex, given that we would enable hypervisor
features very early.

To achieve what you want it would require rewriting of other parts that
are outside of hypervisor framework.

Wei.

> 
> Jan

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