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

[Wei Liu <wl@xen.org>]

On Mon, Dec 30, 2019 at 01:04:33PM +0000, Andrew Cooper wrote:
> 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@microsoft.com>
> > ---
> >  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.

This works for me. Thanks.

Wei.

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