Re: [Xen-devel] [PATCH v3 1/5] x86/hyperv: setup hypercall page

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 05/01/2020 16:47, Wei Liu wrote:
> diff --git a/xen/arch/x86/guest/hyperv/Makefile b/xen/arch/x86/guest/hyperv/Makefile
> index 68170109a9..1a8887d2f4 100644
> --- a/xen/arch/x86/guest/hyperv/Makefile
> +++ b/xen/arch/x86/guest/hyperv/Makefile
> @@ -1 +1,2 @@
> +obj-y += hypercall_page.o
>  obj-y += hyperv.o
> diff --git a/xen/arch/x86/guest/hyperv/hypercall_page.S b/xen/arch/x86/guest/hyperv/hypercall_page.S
> new file mode 100644
> index 0000000000..6d6ab913be
> --- /dev/null
> +++ b/xen/arch/x86/guest/hyperv/hypercall_page.S
> @@ -0,0 +1,21 @@
> +#include <asm/asm_defns.h>
> +#include <asm/page.h>
> +
> +        .section ".text.page_aligned", "ax", @progbits
> +        .p2align PAGE_SHIFT
> +GLOBAL(hv_hypercall_page)
> +        /* Return -1 for "not yet ready" state */
> +        mov -1, %rax
> +        ret
> +1:
> +        /* Fill the rest with `ret` */
> +        .fill PAGE_SIZE - (1b - hv_hypercall_page), 1, 0xc3

If you want to fill with rets, you can do this more simply with:

    .p2lign PAGE_SHIFT, 0xc3

which will do the size calculation for you.

That said, I retract my statement about wanting this in the middle of
.text.  (Sorry.  See below.)

> diff --git a/xen/arch/x86/guest/hyperv/hyperv.c b/xen/arch/x86/guest/hyperv/hyperv.c
> index 8d38313d7a..381be2a68c 100644
> --- a/xen/arch/x86/guest/hyperv/hyperv.c
> +++ b/xen/arch/x86/guest/hyperv/hyperv.c
> @@ -72,6 +72,27 @@ const struct hypervisor_ops *__init hyperv_probe(void)
>      return &ops;
>  }
>  
> +static void __init setup_hypercall_page(void)
> +{
> +    union hv_x64_msr_hypercall_contents hypercall_msr;
> +
> +    rdmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> +    hypercall_msr.enable = 1;
> +    hypercall_msr.guest_physical_address =
> +        __pa(hv_hypercall_page) >> HV_HYP_PAGE_SHIFT;
> +    wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> +}
> +
> +static void __init setup(void)
> +{
> +    setup_hypercall_page();
> +}

The TLFS says that writing enable will fail until the OS identity is
set, which AFACIT, isn't done anywhere in the series.  The whole
sequence is described in "3.13 Establishing the Hypercall Interface"

The locked bit is probably a good idea, but one aspect missing here is
the check to see whether the hypercall page is already enabled, which I
expect is for a kexec crash scenario.

However, the most important point is the one which describes the #GP
properties of the guest trying to modify the page.  This can only be
achieved with an EPT/NPT mapping lacking the W permission, which will
shatter host superpages.   Therefore, putting it in .text is going to be
rather poor, perf wise.

I also note that Xen's implementation of the Viridian hypercall page
doesn't conform to these properties, and wants fixing.  It is going to
need a new kind identification of the page (probably a new p2m type)
which injects #GP if we ever see an EPT_VIOLATION/NPT_FAULT against it.

As for suggestions here, I'm struggling to find any memory map details
exposed in the Viridian interface, and therefore which gfn is best to
choose.  I have a sinking feeling that the answer is ACPI...

~Andrew

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