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

[Wei Liu]

On Tue, Sep 15, 2020 at 12:32:29PM +0200, Vitaly Kuznetsov wrote:
> Wei Liu <wei.liu@xxxxxxxxxx> 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@xxxxxxxxxxxxx>
> > Signed-off-by: Sunil Muthuswamy <sunilmut@xxxxxxxxxxxxx>
> > Signed-off-by: Nuno Das Neves <nudasnev@xxxxxxxxxxxxx>
> > Co-Developed-by: Lillian Grassin-Drake <ligrassi@xxxxxxxxxxxxx>
> > Co-Developed-by: Sunil Muthuswamy <sunilmut@xxxxxxxxxxxxx>
> > Co-Developed-by: Nuno Das Neves <nudasnev@xxxxxxxxxxxxx>
> > Signed-off-by: Wei Liu <wei.liu@xxxxxxxxxx>
> > ---
> >  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.

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
>