Re: [Xen-devel] [PATCH v7 05/10] x86/hyperv: setup hypercall page

["Roger Pau Monné" <roger.pau@citrix.com>]

On Tue, Feb 04, 2020 at 03:36:59PM +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.
> 
> To avoid shattering L0 superpages and treading on any MMIO areas
> residing in low addresses, use the top-most addressable page for that
> purpose. Adjust e820 map accordingly.
> 
> 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@microsoft.com>

Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

> ---
> v7:
> 1. Fix a style issue
> 2. Initialise ID to 0
> 3. Update commit message
> 
> 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..2e20a96f30 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;

Is it expected that the guest ID or the hypercal page is already
setup?

Ie: would virtual firmware setup any of this? (and not clean it up
afterwards)

Thanks, Roger.

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