RE: [PATCH 2/7] x86/hyper-v: fast hypercall implementation

[KY Srinivasan]

> -----Original Message-----
> From: Vitaly Kuznetsov [mailto:vkuznets@xxxxxxxxxx]
> Sent: Friday, April 7, 2017 4:27 AM
> To: devel@xxxxxxxxxxxxxxxxxxxxxx; x86@xxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx; KY Srinivasan <kys@xxxxxxxxxxxxx>;
> Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>; Stephen Hemminger
> <sthemmin@xxxxxxxxxxxxx>; Thomas Gleixner <tglx@xxxxxxxxxxxxx>; Ingo
> Molnar <mingo@xxxxxxxxxx>; H. Peter Anvin <hpa@xxxxxxxxx>; Steven
> Rostedt <rostedt@xxxxxxxxxxx>; Jork Loeser <Jork.Loeser@xxxxxxxxxxxxx>
> Subject: [PATCH 2/7] x86/hyper-v: fast hypercall implementation
> 
> Hyper-V supports 'fast' hypercalls when all parameters are passed through
> registers. Implement an inline version of a simpliest of these calls:
> hypercall with one 8-byte input and no output.
> 
> Proper hypercall input interface (struct hv_hypercall_input) definition is
> added as well.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/mshyperv.h    | 37
> +++++++++++++++++++++++++++++++++++++
>  arch/x86/include/uapi/asm/hyperv.h | 19 +++++++++++++++++++
>  2 files changed, 56 insertions(+)
> 
> diff --git a/arch/x86/include/asm/mshyperv.h
> b/arch/x86/include/asm/mshyperv.h
> index 331e834..9a5f58b 100644
> --- a/arch/x86/include/asm/mshyperv.h
> +++ b/arch/x86/include/asm/mshyperv.h
> @@ -216,6 +216,43 @@ static inline u64 hv_do_hypercall(u64 control, void
> *input, void *output)
>  #endif /* !x86_64 */
>  }
> 
> +/* Fast hypercall with 8 bytes of input and no output */
> +static inline u64 hv_do_fast_hypercall8(u16 code, u64 input1)
> +{
> +	union hv_hypercall_input control = {0};

Defining the hyper-call arguments on the stack can be problematic
if CONFIG_VMAP_STACK is defined - since we are passing the guest
physical address to the hypervisor, we cannot have the arguments
straddle a page boundary.
We have dealt with this issue currently by making sure the arguments are
never on the stack via different means. Perhaps, we can allocate memory
on a per-cpu basis that can be used for this purpose. In fact, this is what I have done
for the   hv_post_message hypercall. We can just rename that page and use it in all
hypercalls - we can just allocate two pages on a per-CPU basis for this purpose
(for input and output parameters).
> +
> +	control.code = code;
> +	control.fast = 1;
> +#ifdef CONFIG_X86_64
> +	{
> +		u64 hv_status;
> +
> +		__asm__ __volatile__("call *%3"
> +				     : "=a" (hv_status)
> +				     : "c" (control.as_uint64), "d" (input1),
> +				       "m" (hv_hypercall_pg)
> +				     : "cc", "r8", "%r9", "%r10", "%r11");
> +		return hv_status;
> +	}
> +#else
> +	{
> +		u32 hv_status_hi, hv_status_lo;
> +
> +		__asm__ __volatile__ ("call *%6"
> +				      : "=d"(hv_status_hi),
> +					"=a"(hv_status_lo) :
> +					"d" (control.as_uint32_hi),
> +					"a" (control.as_uint32_lo),
> +					"c" ((u32)input1),
> +					"b" ((u32)(input1 >> 32)),
> +					"m" (hv_hypercall_pg)
> +				      : "cc");
> +
> +		return hv_status_lo | ((u64)hv_status_hi << 32);
> +	}
> +#endif
> +}
> +
>  void hyperv_init(void);
>  void hyperv_report_panic(struct pt_regs *regs);
>  bool hv_is_hypercall_page_setup(void);
> diff --git a/arch/x86/include/uapi/asm/hyperv.h
> b/arch/x86/include/uapi/asm/hyperv.h
> index 432df4b..c87e900 100644
> --- a/arch/x86/include/uapi/asm/hyperv.h
> +++ b/arch/x86/include/uapi/asm/hyperv.h
> @@ -256,6 +256,25 @@
>  #define HV_PROCESSOR_POWER_STATE_C2		2
>  #define HV_PROCESSOR_POWER_STATE_C3		3
> 
> +/* Hypercall interface */
> +union hv_hypercall_input {
> +	u64 as_uint64;
> +	struct {
> +		__u32 as_uint32_lo;
> +		__u32 as_uint32_hi;
> +	};
> +	struct {
> +		__u64 code:16;
> +		__u64 fast:1;
> +		__u64 varhead_size:10;
> +		__u64 reserved1:5;
> +		__u64 rep_count:12;
> +		__u64 reserved2:4;
> +		__u64 rep_start:12;
> +		__u64 reserved3:4;
> +	};
> +};
> +
>  /* hypercall status code */
>  #define HV_STATUS_SUCCESS			0
>  #define HV_STATUS_INVALID_HYPERCALL_CODE	2
> --
> 2.9.3