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From: Wei Liu <wei.liu@kernel.org>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Wei Liu <wei.liu@kernel.org>,
	Linux on Hyper-V List <linux-hyperv@vger.kernel.org>,
	virtualization@lists.linux-foundation.org,
	Linux Kernel List <linux-kernel@vger.kernel.org>,
	Michael Kelley <mikelley@microsoft.com>,
	Vineeth Pillai <viremana@linux.microsoft.com>,
	Sunil Muthuswamy <sunilmut@microsoft.com>,
	Nuno Das Neves <nudasnev@microsoft.com>,
	Lillian Grassin-Drake <ligrassi@microsoft.com>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH RFC v1 08/18] x86/hyperv: handling hypercall page setup for root
Date: Tue, 15 Sep 2020 11:16:57 +0000	[thread overview]
Message-ID: <20200915111657.boa4cneqjqtmcaaq@liuwe-devbox-debian-v2> (raw)
In-Reply-To: <87pn6njob3.fsf@vitty.brq.redhat.com>

On Tue, Sep 15, 2020 at 01:02:08PM +0200, Vitaly Kuznetsov wrote:
> Wei Liu <wei.liu@kernel.org> writes:
> 
> > On Tue, Sep 15, 2020 at 12:32:29PM +0200, Vitaly Kuznetsov wrote:
> >> Wei Liu <wei.liu@kernel.org> writes:
> >> 
> >> > When Linux is running as the root partition, the hypercall page will
> >> > have already been setup by Hyper-V. Copy the content over to the
> >> > allocated page.
> >> 
> >> And we can't setup a new hypercall page by writing something different
> >> to HV_X64_MSR_HYPERCALL, right?
> >> 
> >
> > My understanding is that we can't, but Sunil can maybe correct me.
> >
> >> >
> >> > The suspend, resume and cleanup paths remain untouched because they are
> >> > not supported in this setup yet.
> >> >
> >> > Signed-off-by: Lillian Grassin-Drake <ligrassi@microsoft.com>
> >> > Signed-off-by: Sunil Muthuswamy <sunilmut@microsoft.com>
> >> > Signed-off-by: Nuno Das Neves <nudasnev@microsoft.com>
> >> > Co-Developed-by: Lillian Grassin-Drake <ligrassi@microsoft.com>
> >> > Co-Developed-by: Sunil Muthuswamy <sunilmut@microsoft.com>
> >> > Co-Developed-by: Nuno Das Neves <nudasnev@microsoft.com>
> >> > Signed-off-by: Wei Liu <wei.liu@kernel.org>
> >> > ---
> >> >  arch/x86/hyperv/hv_init.c | 26 ++++++++++++++++++++++++--
> >> >  1 file changed, 24 insertions(+), 2 deletions(-)
> >> >
> >> > diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> >> > index 0eec1ed32023..26233aebc86c 100644
> >> > --- a/arch/x86/hyperv/hv_init.c
> >> > +++ b/arch/x86/hyperv/hv_init.c
> >> > @@ -25,6 +25,7 @@
> >> >  #include <linux/cpuhotplug.h>
> >> >  #include <linux/syscore_ops.h>
> >> >  #include <clocksource/hyperv_timer.h>
> >> > +#include <linux/highmem.h>
> >> >  
> >> >  /* Is Linux running as the root partition? */
> >> >  bool hv_root_partition;
> >> > @@ -448,8 +449,29 @@ void __init hyperv_init(void)
> >> >  
> >> >  	rdmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> >> >  	hypercall_msr.enable = 1;
> >> > -	hypercall_msr.guest_physical_address = vmalloc_to_pfn(hv_hypercall_pg);
> >> > -	wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> >> > +
> >> > +	if (hv_root_partition) {
> >> > +		struct page *pg;
> >> > +		void *src, *dst;
> >> > +
> >> > +		/*
> >> > +		 * Order is important here. We must enable the hypercall page
> >> > +		 * so it is populated with code, then copy the code to an
> >> > +		 * executable page.
> >> > +		 */
> >> > +		wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> >> > +
> >> > +		pg = vmalloc_to_page(hv_hypercall_pg);
> >> > +		dst = kmap(pg);
> >> > +		src = memremap(hypercall_msr.guest_physical_address << PAGE_SHIFT, PAGE_SIZE,
> >> > +				MEMREMAP_WB);
> >> 
> >> memremap() can fail...
> >
> > And we don't care here, if it fails, we would rather it panic or oops.
> >
> > I was relying on the fact that copying from / to a NULL pointer will
> > cause the kernel to crash. But of course it wouldn't hurt to explicitly
> > panic here.
> >
> >> 
> >> > +		memcpy(dst, src, PAGE_SIZE);
> >> > +		memunmap(src);
> >> > +		kunmap(pg);
> >> > +	} else {
> >> > +		hypercall_msr.guest_physical_address = vmalloc_to_pfn(hv_hypercall_pg);
> >> > +		wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> >> > +	}
> >> 
> >> Why can't we do wrmsrl() for both cases here?
> >> 
> >
> > Because the hypercall page has already been set up when Linux is the
> > root.
> 
> But you already do wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64)
> in 'if (hv_root_partition)' case above, that's why I asked.
> 

You mean extracting wrmsrl to this point?  The ordering matters. See the
comment in the root branch -- we have to enable the page before copying
the content.

What can be done is:

   if (!root) {
       /* some stuff */
   }

   wrmsrl(...)

   if (root) {
       /* some stuff */
   }

This is not looking any better than the existing code.

Wei.

> >
> > I could've tried writing to the MSR again, but because the behaviour
> > here is not documented and subject to change so I didn't bother trying.
> >
> > Wei.
> >
> >> >  
> >> >  	/*
> >> >  	 * Ignore any errors in setting up stimer clockevents
> >> 
> >> -- 
> >> Vitaly
> >> 
> >
> 
> -- 
> Vitaly
> 

  reply	other threads:[~2020-09-16  0:57 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-14 11:27 [PATCH RFC v1 00/18] Introducing Linux root partition support for Microsoft Hypervisor Wei Liu
2020-09-14 11:27 ` [PATCH RFC v1 01/18] asm-generic/hyperv: change HV_CPU_POWER_MANAGEMENT to HV_CPU_MANAGEMENT Wei Liu
2020-09-14 11:27 ` [PATCH RFC v1 02/18] x86/hyperv: detect if Linux is the root partition Wei Liu
2020-09-14 11:27 ` [PATCH RFC v1 03/18] Drivers: hv: vmbus: skip VMBus initialization if Linux is root Wei Liu
2020-09-14 11:27 ` [PATCH RFC v1 04/18] iommu/hyperv: don't setup IRQ remapping when running as root Wei Liu
2020-09-18  9:12   ` Joerg Roedel
2020-09-14 11:27 ` [PATCH RFC v1 05/18] clocksource/hyperv: use MSR-based access if " Wei Liu
2020-09-15 10:10   ` Vitaly Kuznetsov
2020-09-15 10:32     ` Wei Liu
2020-09-14 11:27 ` [PATCH RFC v1 06/18] x86/hyperv: allocate output arg pages if required Wei Liu
2020-09-15 10:16   ` Vitaly Kuznetsov
2020-09-15 12:43     ` Wei Liu
2020-09-16 15:42       ` Wei Liu
2020-09-16 16:10         ` Vitaly Kuznetsov
2020-09-14 11:27 ` [PATCH RFC v1 07/18] x86/hyperv: extract partition ID from Microsoft Hypervisor if necessary Wei Liu
2020-09-15 10:27   ` Vitaly Kuznetsov
2020-09-16 16:32     ` Wei Liu
2020-10-27 12:19       ` Wei Liu
2020-09-14 11:27 ` [PATCH RFC v1 08/18] x86/hyperv: handling hypercall page setup for root Wei Liu
2020-09-15 10:32   ` Vitaly Kuznetsov
2020-09-15 10:37     ` Wei Liu
2020-09-15 11:02       ` Vitaly Kuznetsov
2020-09-15 11:16         ` Wei Liu [this message]
2020-09-15 11:23           ` Vitaly Kuznetsov
2020-09-15 11:27             ` Wei Liu
2020-09-16 21:34       ` [EXTERNAL] " Sunil Muthuswamy
2020-09-17 11:06         ` Vitaly Kuznetsov
2020-09-14 11:59 ` [PATCH RFC v1 09/18] x86/hyperv: provide a bunch of helper functions Wei Liu
2020-09-15 11:00   ` Vitaly Kuznetsov
2020-10-27 13:10     ` Wei Liu
2020-09-14 11:59 ` [PATCH RFC v1 10/18] x86/hyperv: implement and use hv_smp_prepare_cpus Wei Liu
2020-09-15 11:14   ` Vitaly Kuznetsov
2020-10-27 13:47     ` Wei Liu
2020-10-27 13:56       ` Wei Liu
2020-09-14 11:59 ` [PATCH RFC v1 11/18] asm-generic/hyperv: update hv_msi_entry Wei Liu
2020-09-14 11:59 ` [PATCH RFC v1 12/18] asm-generic/hyperv: update hv_interrupt_entry Wei Liu
2020-10-01 14:33   ` Rob Herring
2020-09-14 11:59 ` [PATCH RFC v1 13/18] asm-generic/hyperv: introduce hv_device_id and auxiliary structures Wei Liu
2020-09-15 11:16   ` Vitaly Kuznetsov
2020-09-15 11:59     ` Wei Liu
2020-09-14 11:59 ` [PATCH RFC v1 14/18] asm-generic/hyperv: import data structures for mapping device interrupts Wei Liu
2020-09-14 11:59 ` [PATCH RFC v1 15/18] x86/apic/msi: export pci_msi_get_hwirq Wei Liu
2020-09-14 11:59 ` [PATCH RFC v1 16/18] x86/hyperv: implement MSI domain for root partition Wei Liu
2020-09-14 11:59 ` [PATCH RFC v1 17/18] x86/ioapic: export a few functions and data structures via io_apic.h Wei Liu
2020-09-14 11:59 ` [PATCH RFC v1 18/18] x86/hyperv: handle IO-APIC when running as root Wei Liu

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