Re: [PATCH v2 5/9] x86/hyperv: Mark ACPI wakeup mailbox page as private

[Saurabh Singh Sengar]

On Mon, Sep 02, 2024 at 03:35:18AM +0000, Michael Kelley wrote:
> From: Yunhong Jiang <yunhong.jiang@xxxxxxxxxxxxxxx> Sent: Friday, August 23, 2024 4:23 PM
> > 
> > Current code maps MMIO devices as shared (decrypted) by default in a
> > confidential computing VM. However, the wakeup mailbox must be accessed
> > as private (encrypted) because it's accessed by the OS and the firmware,
> > both are in the guest's context and encrypted. Set the wakeup mailbox
> > range as private explicitly.
> > 
> > Signed-off-by: Yunhong Jiang <yunhong.jiang@xxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/hyperv/hv_vtl.c | 16 ++++++++++++++++
> >  1 file changed, 16 insertions(+)
> > 
> > diff --git a/arch/x86/hyperv/hv_vtl.c b/arch/x86/hyperv/hv_vtl.c
> > index 04775346369c..987a6a1200b0 100644
> > --- a/arch/x86/hyperv/hv_vtl.c
> > +++ b/arch/x86/hyperv/hv_vtl.c
> > @@ -22,10 +22,26 @@ static bool __init hv_vtl_msi_ext_dest_id(void)
> >  	return true;
> >  }
> > 
> > +static inline bool within_page(u64 addr, u64 start)
> > +{
> > +	return addr >= start && addr < (start + PAGE_SIZE);
> > +}
> > +
> > +/*
> > + * The ACPI wakeup mailbox are accessed by the OS and the BIOS, both are in the
> > + * guest's context, instead of the hypervisor/VMM context.
> > + */
> > +static bool hv_is_private_mmio_tdx(u64 addr)
> > +{
> > +	return wakeup_mailbox_addr && within_page(addr, wakeup_mailbox_addr);
> > +}
> > +
> >  void __init hv_vtl_init_platform(void)
> >  {
> >  	pr_info("Linux runs in Hyper-V Virtual Trust Level\n");
> > 
> > +	if (hv_isolation_type_tdx())
> > +		x86_platform.hyper.is_private_mmio = hv_is_private_mmio_tdx;
> 
> hv_vtl_init_platform() is unconditionally called in
> ms_hyperv_init_platform(). So in the case of a normal TDX guest
> running with a paravisor on Hyper-V, the above code will overwrite
> the is_private_mmio function that was set in hv_vtom_init(). Then
> the mapping of the emulated IOAPIC and TPM provided by the
> paravisor won't be correct.
> 
> Michael

non-VTL Hyper-V platforms are expected to disable CONFIG_HYPERV_VTL_MODE,
that means for a normal TDX guest hv_vtl_init_platform will be an empty
stub. Have I missed anything ?

- Saurabh