Re: [PATCH RFC v1 08/18] x86/hyperv: handling hypercall page setup for root

From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Wei Liu <wei.liu@kernel.org>
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 13:02:08 +0200	[thread overview]
Message-ID: <87pn6njob3.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20200915103710.cqmdvzh5lys4wsqo@liuwe-devbox-debian-v2>

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.

>
> 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