Re: [Xen-devel] [PATCH] x86/p2m: drop p2m_access_t parameter from set_mmio_p2m_entry()

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Feb 07, 2020 at 09:08:15AM +0100, Jan Beulich wrote:
> On 06.02.2020 16:20, Jan Beulich wrote:
> > --- a/xen/arch/x86/hvm/vmx/vmx.c
> > +++ b/xen/arch/x86/hvm/vmx/vmx.c
> > @@ -3037,9 +3037,8 @@ static int vmx_alloc_vlapic_mapping(stru
> >      share_xen_page_with_guest(pg, d, SHARE_rw);
> >      d->arch.hvm.vmx.apic_access_mfn = mfn;
> >  
> > -    return set_mmio_p2m_entry(d, paddr_to_pfn(APIC_DEFAULT_PHYS_BASE), mfn,
> > -                              PAGE_ORDER_4K,
> > -                              p2m_get_hostp2m(d)->default_access);
> > +    return set_mmio_p2m_entry(d, gaddr_to_gfn(APIC_DEFAULT_PHYS_BASE), mfn,
> > +                              PAGE_ORDER_4K);
> >  }
> 
> Upon 2nd thought - does this really want to use default access?
> Execute permission for this page looks a little suspicious.
> Isn't it the case that this page doesn't (normally?) get
> accessed at all, and instead its address serves as an indicator
> to the CPU? (I even vaguely recall it having been considered to
> consolidate this, to e.g. a single page per domain.) In which
> case even p2m_access_n might be good enough?

Hm, I think p2m_access_n is not enough, as that would trigger an EPT
fault which has preference over the APIC access VM exit (see 29.4.1
Priority of APIC-Access VM Exits).

I think setting it to p2m_access_rw should be enough, and we would get
EPT faults when trying to execute from APIC page.

Roger.

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