linux-kernel - RE: [PATCH v2 4/4] drivers/clocksource/hyper-v: Add TSC page support for root partition

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Date:   Thu, 3 Nov 2022 17:22:34 +0000
From:   "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
To:     Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>
CC:     Stanislav Kinsburskiy <stanislav.kinsburskiy@...il.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 4/4] drivers/clocksource/hyper-v: Add TSC page support
 for root partition

From: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com> Sent: Thursday, November 3, 2022 10:01 AM
> 
> O Wed, Nov 02, 2022 at 11:36:45PM +0000, Michael Kelley (LINUX) wrote:
> > From: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com> Sent: Wednesday,
> November 2, 2022 3:08 PM
> > >
> > > Microsoft Hypervisor root partition has to map the TSC page specified
> > > by the hypervisor, instead of providing the page to the hypervisor like
> > > it's done in the guest partitions.
> > >
> > > However, it's too early to map the page when the clock is initialized, so, the
> > > actual mapping is happening later.
> > >
> > > Signed-off-by: Stanislav Kinsburskiy <stanislav.kinsburskiy@...il.com>
> > > CC: "K. Y. Srinivasan" <kys@...rosoft.com>
> > > CC: Haiyang Zhang <haiyangz@...rosoft.com>
> > > CC: Wei Liu <wei.liu@...nel.org>
> > > CC: Dexuan Cui <decui@...rosoft.com>
> > > CC: Thomas Gleixner <tglx@...utronix.de>
> > > CC: Ingo Molnar <mingo@...hat.com>
> > > CC: Borislav Petkov <bp@...en8.de>
> > > CC: Dave Hansen <dave.hansen@...ux.intel.com>
> > > CC: x86@...nel.org
> > > CC: "H. Peter Anvin" <hpa@...or.com>
> > > CC: Daniel Lezcano <daniel.lezcano@...aro.org>
> > > CC: linux-hyperv@...r.kernel.org
> > > CC: linux-kernel@...r.kernel.org
> > > ---
> > >  arch/x86/hyperv/hv_init.c          |    2 ++
> > >  drivers/clocksource/hyperv_timer.c |   37 +++++++++++++++++++++++++++------
> ---
> > >  include/clocksource/hyperv_timer.h |    1 +
> > >  3 files changed, 31 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> > > index f49bc3ec76e6..89954490af93 100644
> > > --- a/arch/x86/hyperv/hv_init.c
> > > +++ b/arch/x86/hyperv/hv_init.c
> > > @@ -464,6 +464,8 @@ void __init hyperv_init(void)
> > >  		BUG_ON(!src);
> > >  		memcpy_to_page(pg, 0, src, HV_HYP_PAGE_SIZE);
> > >  		memunmap(src);
> > > +
> > > +		hv_remap_tsc_clocksource();
> > >  	} else {
> > >  		hypercall_msr.guest_physical_address =
> > > vmalloc_to_pfn(hv_hypercall_pg);
> > >  		wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> > > diff --git a/drivers/clocksource/hyperv_timer.c
> b/drivers/clocksource/hyperv_timer.c
> > > index 635c14c1e3bf..ec76303b2a76 100644
> > > --- a/drivers/clocksource/hyperv_timer.c
> > > +++ b/drivers/clocksource/hyperv_timer.c
> > > @@ -508,9 +508,6 @@ static bool __init hv_init_tsc_clocksource(void)
> > >  	if (!(ms_hyperv.features & HV_MSR_REFERENCE_TSC_AVAILABLE))
> > >  		return false;
> > >
> > > -	if (hv_root_partition)
> > > -		return false;
> > > -
> > >  	/*
> > >  	 * If Hyper-V offers TSC_INVARIANT, then the virtualized TSC correctly
> > >  	 * handles frequency and offset changes due to live migration,
> > > @@ -528,16 +525,22 @@ static bool __init hv_init_tsc_clocksource(void)
> > >  	}
> > >
> > >  	hv_read_reference_counter = read_hv_clock_tsc;
> > > -	tsc_pfn = __phys_to_pfn(virt_to_phys(tsc_page));
> > >
> > >  	/*
> > > -	 * The Hyper-V TLFS specifies to preserve the value of reserved
> > > -	 * bits in registers. So read the existing value, preserve the
> > > -	 * low order 12 bits, and add in the guest physical address
> > > -	 * (which already has at least the low 12 bits set to zero since
> > > -	 * it is page aligned). Also set the "enable" bit, which is bit 0.
> > > +	 * TSC page mapping works differently in root and guest partitions.
> > > +	 * - In guest partition the guest PFN has to be passed to the
> > > +	 *   hypervisor.
> > > +	 * - In root partition it's other way around: it has to map the PFN
> > > +	 *   provided by the hypervisor.
> > > +	 *   But it can't be mapped right here as it's too early and MMU isn't
> > > +	 *   ready yet. So, we only set the enable bit here and will remap the
> > > +	 *   page later in hv_remap_tsc_clocksource().
> > >  	 */
> > >  	tsc_msr.as_uint64 = hv_get_register(HV_REGISTER_REFERENCE_TSC);
> > > +	if (hv_root_partition)
> > > +		tsc_pfn = tsc_msr.pfn;
> > > +	else
> > > +		tsc_pfn = __phys_to_pfn(virt_to_phys(tsc_page));
> >
> > Same problem here with setting tsc_pfn to a guest PFN, which may be
> > Different from what Hyper-V is expecting as a PFN two lines below.  I know
> > the above line was just carried over from Anirudh's previous patch set,
> > but I was thinking you would fix this issue. :-)
> >
> 
> Fair call. I guess Anirudh has addressed it himself, so I'm going to
> rebase on his fix.

Sounds good.

> 
> > >  	tsc_msr.enable = 1;
> > >  	tsc_msr.pfn = tsc_pfn;
> > >  	hv_set_register(HV_REGISTER_REFERENCE_TSC, tsc_msr.as_uint64);
> > > @@ -572,3 +575,19 @@ void __init hv_init_clocksource(void)
> > >  	hv_sched_clock_offset = hv_read_reference_counter();
> > >  	hv_setup_sched_clock(read_hv_sched_clock_msr);
> > >  }
> > > +
> > > +void __init hv_remap_tsc_clocksource(void)
> > > +{
> > > +	if (!(ms_hyperv.features & HV_MSR_REFERENCE_TSC_AVAILABLE))
> > > +		return;
> > > +
> > > +	if (!hv_root_partition) {
> > > +		WARN(1, "%s: attempt to remap TSC page in guest partition\n",
> > > +		     __func__);
> > > +		return;
> > > +	}
> > > +
> > > +	tsc_page = memremap(__pfn_to_phys(tsc_pfn), sizeof(tsc_pg), MEMREMAP_WB);
> >
> > Note that use of __pfn_to_phys() is at risk of being wrong depending on whether
> > you decide to keep a guest PFN or a Hyper-V PFN in tsc_pfn.
> >
> 
> It's Hyperv-V PFN that is stored in the variable (to match the MSR value for the root
> partition).  I guess this approach will workd regardless of the guest page size.

I agree that it's best for tsc_pfn to contain a Hyper-V PFN.  But if that's the case,
the above use of __pfn_to_phys() won't work since it is expecting a guest PFN
as input.

Michael