Re: [Xen-devel] [PATCH v7 05/10] x86/hyperv: setup hypercall page

[Wei Liu <wl@xxxxxxx>]

On Wed, Feb 05, 2020 at 03:04:16PM +0100, Roger Pau Monné wrote:
[...]
> > +static void __init setup_hypercall_page(void)
> > +{
> > +    union hv_x64_msr_hypercall_contents hypercall_msr;
> > +    union hv_guest_os_id guest_id;
> > +    unsigned long mfn;
> > +
> > +    BUILD_BUG_ON(HV_HYP_PAGE_SHIFT != PAGE_SHIFT);
> > +
> > +    rdmsrl(HV_X64_MSR_GUEST_OS_ID, guest_id.raw);
> > +    if ( !guest_id.raw )
> > +    {
> > +        guest_id.raw = generate_guest_id();
> > +        wrmsrl(HV_X64_MSR_GUEST_OS_ID, guest_id.raw);
> > +    }
> > +
> > +    rdmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> > +    if ( !hypercall_msr.enable )
> > +    {
> > +        mfn = HV_HCALL_MFN;
> > +        hypercall_msr.enable = 1;
> > +        hypercall_msr.guest_physical_address = mfn;
> > +        wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> > +    }
> > +    else
> > +        mfn = hypercall_msr.guest_physical_address;
> 
> Is it expected that the guest ID or the hypercal page is already
> setup?
> 
> Ie: would virtual firmware setup any of this? (and not clean it up
> afterwards)

There is no definitive answer to why/when this happens in TLFS, but it
does require guest to respect that is already in the MSR.

Wei.

> 
> Thanks, Roger.

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