Re: [Xen-devel] [PATCH 5/8] x86/hyperv: provide Hyper-V hypercall functions

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 29/12/2019 18:33, Wei Liu wrote:
> These functions will be used later to make hypercalls to Hyper-V.
>
> I couldn't find reference in TLFS that Hyper-V clobbers flags and
> r9-r11, but Linux's commit message says it does. Err on the safe side.
>
> Signed-off-by: Wei Liu <liuwe@xxxxxxxxxxxxx>
> ---
>  xen/include/asm-x86/guest/hyperv-hypercall.h | 105 +++++++++++++++++++
>  1 file changed, 105 insertions(+)
>  create mode 100644 xen/include/asm-x86/guest/hyperv-hypercall.h
>
> diff --git a/xen/include/asm-x86/guest/hyperv-hypercall.h 
> b/xen/include/asm-x86/guest/hyperv-hypercall.h
> new file mode 100644
> index 0000000000..6017123be5
> --- /dev/null
> +++ b/xen/include/asm-x86/guest/hyperv-hypercall.h
> @@ -0,0 +1,105 @@
> +/******************************************************************************
> + * asm-x86/guest/hyperv-hypercall.h
> + *
> + * This program is free software; you can redistribute it and/or
> + * modify it under the terms and conditions of the GNU General Public
> + * License, version 2, as published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> + * General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public
> + * License along with this program; If not, see 
> <http://www.gnu.org/licenses/>.
> + *
> + * Copyright (c) 2019 Microsoft.
> + */
> +
> +#ifndef __X86_HYPERV_HYPERCALL_H__
> +#define __X86_HYPERV_HYPERCALL_H__
> +
> +#include <xen/types.h>
> +
> +#include <asm/asm_defns.h>
> +#include <asm/guest/hyperv-tlfs.h>
> +#include <asm/page.h>
> +
> +extern void *hv_hypercall;
> +
> +static inline uint64_t hv_do_hypercall(uint64_t control, paddr_t input, 
> paddr_t output)
> +{
> +    uint64_t status;
> +
> +    if ( !hv_hypercall )
> +        return ~0ULL;
> +
> +    asm volatile ("mov %[output], %%r8\n"
> +                  "call *%[hypercall_page]"
> +                  : "=a" (status), "+c" (control),
> +                    "+d" (input) ASM_CALL_CONSTRAINT
> +                  : [output] "rm" (output),
> +                    [hypercall_page] "m" (hv_hypercall)
> +                  : "cc", "memory", "r8", "r9", "r10", "r11");
> +
> +    return status;
> +}

Indirect calls are expensive these days due to retpoline/IBRS, and in
this case, unnecessary.

You want something like:

asm ( ".pushsection \".text.page_aligned\", \"ax\", @progbits\n\t"
      ".align 4096\n\t"
      ".globl hyperv_hypercall\n\t"
      "hyperv_hypercall:\n\t"
      "mov -1, %rax\n\t"
      "ret\n\t"
      ".align 4096;\n\t" );

Which will put one page worth of space in .text.page_aligned (so it gets
mapped executable), at a location the linker can evaluate (so you can
use a direct call, and the disassembly will be easier to follow), which
is initialised to the "not ready yet" code so you don't need a runtime
check in every hypercall that you didn't get the order of initialisation
wrong at boot.

Alternatively, initialise to ud2 if some form of console can be reliably
be arranged to work from the very start.

~Andrew

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