Re: [uml-devel] um: PTRACE_SETREGSET failure with XSTATE on Kabylake CPU

[Richard W. <ri...@no...>]

Thomas, Natale,

Am 21.06.2017 um 11:44 schrieb Thomas Meyer:
>> It is funny to see that this problem was firstly reported here [1] in
>> February 2017 without being considered until someone else bought a
>> new
>> laptop :)

sometimes mails/issues get lost...

> Yes, I like my new laptop :-)
> 
>> Anyway, thank you for digging into this; my temporary workaround at
>> the time was to use always the *_i387_registers functions.
> 
> Oops, there is the complete thread with the same problem. But sorry I
> don't follow uml-user, just uml-devel :-(

We really should merge these lists. ;-\

> As a quick fix you can try this:
> 
> diff --git a/arch/x86/um/os-Linux/registers.c b/arch/x86/um/os-Linux/registers.c
> index 00f54a91bb4b..6eac8220ab29 100644
> --- a/arch/x86/um/os-Linux/registers.c
> +++ b/arch/x86/um/os-Linux/registers.c
> @@ -30,7 +30,7 @@ int save_fp_registers(int pid, unsigned long *fp_regs)
>  
>  	if (have_xstate_support) {
>  		iov.iov_base = fp_regs;
> -		iov.iov_len = sizeof(struct _xstate);
> +		iov.iov_len = HOST_FP_SIZE * sizeof(unsigned long);
>  		if (ptrace(PTRACE_GETREGSET, pid, NT_X86_XSTATE, &iov) < 0)
>  			return -errno;
>  		return 0;
> @@ -49,10 +49,9 @@ int restore_i387_registers(int pid, unsigned long *fp_regs)
>  int restore_fp_registers(int pid, unsigned long *fp_regs)
>  {
>  	struct iovec iov;
> -
>  	if (have_xstate_support) {
>  		iov.iov_base = fp_regs;
> -		iov.iov_len = sizeof(struct _xstate);
> +		iov.iov_len = HOST_FP_SIZE * sizeof(unsigned long);
>  		if (ptrace(PTRACE_SETREGSET, pid, NT_X86_XSTATE, &iov) < 0)
>  			return -errno;
>  		return 0;
> @@ -126,7 +125,7 @@ void arch_init_registers(int pid)
>  	struct iovec iov;
>  
>  	iov.iov_base = &fp_regs;
> -	iov.iov_len = sizeof(struct _xstate);
> +	iov.iov_len = HOST_FP_SIZE * sizeof(unsigned long);
>  	if (ptrace(PTRACE_GETREGSET, pid, NT_X86_XSTATE, &iov) == 0)
>  		have_xstate_support = 1;
>  }
> diff --git a/arch/x86/um/user-offsets.c b/arch/x86/um/user-offsets.c
> index cb3c22370cf5..9dccbbbf2fd1 100644
> --- a/arch/x86/um/user-offsets.c
> +++ b/arch/x86/um/user-offsets.c
> @@ -50,7 +50,7 @@ void foo(void)
>  	DEFINE(HOST_GS, GS);
>  	DEFINE(HOST_ORIG_AX, ORIG_EAX);
>  #else
> -	DEFINE(HOST_FP_SIZE, sizeof(struct _xstate) / sizeof(unsigned long));
> +	DEFINE_LONGS(HOST_FP_SIZE, 2688);
>  	DEFINE_LONGS(HOST_BX, RBX);
>  	DEFINE_LONGS(HOST_CX, RCX);
>  	DEFINE_LONGS(HOST_DI, RDI);
> 
> 
> a better fix would be to make the fp regs in struct uml_pt_regs dynamic
> depending on the current kernel idea of the xsave area size.

And we should consider a mechanism to save/restore the full regset only when needed.
Otherwise the context switch is even more slower.

Thanks,
//richard