Re: [Xen-devel] [PATCH v6 05/11] x86/hyperv: setup hypercall page

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Jan 31, 2020 at 05:49:24PM +0000, Wei Liu wrote:
> Hyper-V uses a technique called overlay page for its hypercall page. It
> will insert a backing page to the guest when the hypercall functionality
> is enabled. That means we can use a page that is not backed by real
> memory for hypercall page.
> 
> Use the top-most addressable page for that purpose. Adjust e820 map
> accordingly.

Can you add this is done to avoid page shattering and to make sure
Xen isn't overwriting any MMIO area which might be present at lower
addresses?

> 
> We also need to register Xen's guest OS ID to Hyper-V. Use 0x3 as the
> vendor ID. Fix the comment in hyperv-tlfs.h while at it.
> 
> Signed-off-by: Wei Liu <liuwe@xxxxxxxxxxxxx>
> ---
> v6:
> 1. Use hv_guest_os_id
> 2. Use new e820_fixup hook
> 3. Add a BUILD_BUG_ON
> 
> v5:
> 1. use hypervisor_reserve_top_pages
> 2. add a macro for hypercall page mfn
> 3. address other misc comments
> 
> v4:
> 1. Use fixmap
> 2. Follow routines listed in TLFS
> ---
>  xen/arch/x86/guest/hyperv/hyperv.c      | 69 +++++++++++++++++++++++--
>  xen/include/asm-x86/guest/hyperv-tlfs.h |  5 +-
>  xen/include/asm-x86/guest/hyperv.h      |  3 ++
>  3 files changed, 72 insertions(+), 5 deletions(-)
> 
> diff --git a/xen/arch/x86/guest/hyperv/hyperv.c 
> b/xen/arch/x86/guest/hyperv/hyperv.c
> index 8d38313d7a..7c2a96d70e 100644
> --- a/xen/arch/x86/guest/hyperv/hyperv.c
> +++ b/xen/arch/x86/guest/hyperv/hyperv.c
> @@ -19,15 +19,27 @@
>   * Copyright (c) 2019 Microsoft.
>   */
>  #include <xen/init.h>
> +#include <xen/version.h>
>  
> +#include <asm/fixmap.h>
>  #include <asm/guest.h>
>  #include <asm/guest/hyperv-tlfs.h>
> +#include <asm/processor.h>
>  
>  struct ms_hyperv_info __read_mostly ms_hyperv;
>  
> -static const struct hypervisor_ops ops = {
> -    .name = "Hyper-V",
> -};
> +static uint64_t generate_guest_id(void)
> +{
> +    union hv_guest_os_id id;
> +
> +    id.vendor = HV_XEN_VENDOR_ID;
> +    id.major = xen_major_version();
> +    id.minor = xen_minor_version();
> +
> +    return id.raw;
> +}
> +
> +static const struct hypervisor_ops ops;
>  
>  const struct hypervisor_ops *__init hyperv_probe(void)
>  {
> @@ -72,6 +84,57 @@ 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;
> +    union hv_guest_os_id guest_id;
> +    unsigned long mfn;
> +
> +    BUILD_BUG_ON(HV_HYP_PAGE_SHIFT != PAGE_SHIFT);
> +
> +    rdmsrl(HV_X64_MSR_GUEST_OS_ID, guest_id.raw);
> +    if ( !guest_id.raw )
> +    {
> +        guest_id.raw = generate_guest_id();
> +        wrmsrl(HV_X64_MSR_GUEST_OS_ID, guest_id.raw);
> +    }
> +
> +    rdmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> +    if ( !hypercall_msr.enable )
> +    {
> +        mfn = HV_HCALL_MFN;
> +        hypercall_msr.enable = 1;
> +        hypercall_msr.guest_physical_address = mfn;
> +        wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> +    } else {
> +        mfn = hypercall_msr.guest_physical_address;
> +    }
> +
> +    rdmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> +    BUG_ON(!hypercall_msr.enable);
> +
> +    set_fixmap_x(FIX_X_HYPERV_HCALL, mfn << PAGE_SHIFT);
> +}
> +
> +static void __init setup(void)
> +{
> +    setup_hypercall_page();
> +}
> +
> +static void __init e820_fixup(struct e820map *e820)
> +{
> +    uint64_t s = HV_HCALL_MFN << PAGE_SHIFT;
> +
> +    if ( !e820_add_range(e820, s, s + PAGE_SIZE, E820_RESERVED) )

I think end should be s + PAGE_SIZE - 1, or else it expands across two
pages?

Thanks, Roger.

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