Received-SPF: pass (google.com: domain of linux-kernel+bounces-187436-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.80.249 as permitted sender) client-ip=147.75.80.249;
Date: Thu, 23 May 2024 15:06:23 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To: Dexuan Cui <decui@microsoft.com>
Cc: x86@kernel.org, linux-coco@lists.linux.dev, bp@alien8.de, 
	dave.hansen@intel.com, dave.hansen@linux.intel.com, haiyangz@microsoft.com, 
	hpa@zytor.com, kys@microsoft.com, luto@kernel.org, mingo@redhat.com, 
	peterz@infradead.org, sathyanarayanan.kuppuswamy@linux.intel.com, tglx@linutronix.de, 
	wei.liu@kernel.org, Jason@zx2c4.com, mhklinux@outlook.com, thomas.lendacky@amd.com, 
	tytso@mit.edu, ardb@kernel.org, linux-hyperv@vger.kernel.org, 
	linux-kernel@vger.kernel.org, Tianyu.Lan@microsoft.com, 
	Elena Reshetova <elena.reshetova@intel.com>
Subject: Re: [RFC PATCH] clocksource: hyper-v: Enable the tsc_page for a TDX
 VM in TD mode
Message-ID: <7yos4yh6te7zcwga3swddpyjyxlif2c5vqad2rouwf7euw47df@jvouxfoakct6>
References: <20240523022441.20879-1-decui@microsoft.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20240523022441.20879-1-decui@microsoft.com>

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