Re: [RFC PATCH] clocksource: hyper-v: Enable the tsc_page for a TDX VM in TD mode

[Kirill A. Shutemov]

On Wed, May 22, 2024 at 07:24:41PM -0700, Dexuan Cui wrote:
> A TDX VM on Hyper-V may run in TD mode or Partitioned TD mode (L2). For
> the former, the VM has not enabled the Hyper-V TSC page (which is defined
> in drivers/clocksource/hyperv_timer.c: "... tsc_pg __bss_decrypted ...")
> because, for such a VM, the hypervisor requires that the page should be
> shared, but currently the __bss_decrypted is not working for such a VM
> yet.

I don't see how it is safe. It opens guest clock for manipulations form
VMM. Could you elaborate on security implications?

> Hyper-V TSC page can work as a clocksource device similar to KVM pv
> clock, and it's also used by the Hyper-V timer code to get the current
> time: see hv_init_tsc_clocksource(), which sets the global function
> pointer hv_read_reference_counter to read_hv_clock_tsc(); when
> Hyper-V TSC page is not enabled, hv_read_reference_counter defaults to
> be drivers/hv/hv_common.c: __hv_read_ref_counter(), which is suboptimal
> as it uses the slow MSR interface to get the time info.

Why can't the guest just read the TSC directly? Why do we need the page?
I am confused.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov