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

[Michael Kelley]

From: Saurabh Singh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx> Sent: Monday, September 2, 2024 11:39 AM
> 
> 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 ?
> 

Ah, you are right. My concern is misplaced and can be ignored. :-)

Michael