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

[Vitaly Kuznetsov]

KY Srinivasan <kys@xxxxxxxxxxxxx> writes:

>> -----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).

This is fast hypercall and not the normal one - we pass control to
register so we're fine here (and I'd actually expect the compiler to put
it to the propper register in the first place.


>> +
>> +	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

-- 
  Vitaly