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

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
>

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