Re: [Xen-devel] [PATCH v2 10/18] xen: setup hypercall page for PVH

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Tue, Oct 09, 2018 at 01:03:09PM +0200, Juergen Gross wrote:
> Add the needed code to setup the hypercall page for calling into the
> Xen hypervisor.
>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
>  grub-core/kern/i386/xen/pvh.c | 70 
> +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 70 insertions(+)
>
> diff --git a/grub-core/kern/i386/xen/pvh.c b/grub-core/kern/i386/xen/pvh.c
> index 182ef95f9..c1b1cf8db 100644
> --- a/grub-core/kern/i386/xen/pvh.c
> +++ b/grub-core/kern/i386/xen/pvh.c
> @@ -20,14 +20,84 @@
>  #include <grub/misc.h>
>  #include <grub/memory.h>
>  #include <grub/mm.h>
> +#include <grub/i386/cpuid.h>
>  #include <grub/xen.h>
>  #include <grub/machine/kernel.h>
>
>  grub_uint64_t grub_rsdp_addr;
>
> +static struct { char _entry[32]; } hypercall_page[128]
> +  __attribute__ ((aligned (GRUB_XEN_PAGE_SIZE)));
> +
> +static grub_uint32_t xen_cpuid_base;
> +
> +static void
> +grub_xen_early_halt (void)
> +{
> +  asm volatile ("hlt");
> +}
> +
> +static void
> +grub_xen_cpuid_base (void)
> +{
> +  grub_uint32_t base, eax, signature[3];
> +
> +  for (base = 0x40000000; base < 0x40010000; base += 0x100)
> +    {
> +      grub_cpuid (base, eax, signature[0], signature[1], signature[2]);
> +      if (!grub_memcmp ("XenVMMXenVMM", signature, 12) && (eax - base) >= 2)
> +     {
> +       xen_cpuid_base = base;
> +       return;
> +     }
> +    }
> +
> +  grub_xen_early_halt ();
> +}
> +
> +static void
> +grub_xen_setup_hypercall_page (void)
> +{
> +  grub_uint32_t msr, pfn, eax, ebx, ecx, edx;
> +
> +  grub_cpuid (xen_cpuid_base + 2, eax, ebx, ecx, edx);
> +  msr = ebx;
> +  pfn = (grub_uint32_t) (&hypercall_page[0]);
> +
> +  asm volatile ("wrmsr" : : "c" (msr), "a" (pfn), "d" (0) : "memory");
> +}
> +
> +int
> +grub_xen_hypercall (grub_uint32_t callno, grub_uint32_t a0,
> +                 grub_uint32_t a1, grub_uint32_t a2,
> +                 grub_uint32_t a3, grub_uint32_t a4,
> +                 grub_uint32_t a5 __attribute__ ((unused)))
> +{
> +  register unsigned long __res  asm("eax");
> +  register unsigned long __arg0 asm("ebx") = __arg0;
> +  register unsigned long __arg1 asm("ecx") = __arg1;
> +  register unsigned long __arg2 asm("edx") = __arg2;
> +  register unsigned long __arg3 asm("esi") = __arg3;
> +  register unsigned long __arg4 asm("edi") = __arg4;

Why do we need this play with registers. Does not asm below
work with "a", "b", ... like above?

> +
> +  __arg0 = a0;
> +  __arg1 = a1;
> +  __arg2 = a2;
> +  __arg3 = a3;
> +  __arg4 = a4;
> +  asm volatile ("call *%[callno]"
> +             : "=r" (__res), "+r" (__arg0), "+r" (__arg1), "+r" (__arg2),
> +               "+r" (__arg3), "+r" (__arg4)
> +             : [callno] "a" (&hypercall_page[callno])
> +             : "memory");

Daniel

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