linux-kernel - Re: [PATCH v2] um: Allow building and running on older hosts

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Date:   Thu, 25 May 2017 21:51:53 +0200
From:   Thomas Meyer <thomas@...3r.de>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     linux-kernel@...r.kernel.org, elicooper@....com,
        Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "open list:USER-MODE LINUX (UML)" 
        <user-mode-linux-devel@...ts.sourceforge.net>,
        "open list:USER-MODE LINUX (UML)" 
        <user-mode-linux-user@...ts.sourceforge.net>
Subject: Re: [PATCH v2] um: Allow building and running on older hosts


> Am 25.05.2017 um 20:36 schrieb Florian Fainelli <f.fainelli@...il.com>:
> 
> Commit a78ff1112263 ("um: add extended processor state save/restore
> support") and b6024b21fec8 ("um: extend fpstate to _xstate to support
> YMM registers") forced the use of the x86 FP _xstate and
> PTRACE_GETREGSET/SETREGSET. On older hosts, we would neither be able to
> build UML nor run it anymore with these two commits applied because we
> don't have definitions for struct _xstate nor these two ptrace requests.

Btw. what soy you mean with older host? I need to revert two commits to make an UML kernel work again on a Fedora 25 installation.

> 
> We can determine at build time which fp context structure to check
> against, just like we can keep using the old i387 fp save/restore if
> PTRACE_GETRESET/SETREGSET are not defined.
> 
> Fixes: a78ff1112263 ("um: add extended processor state save/restore support")
> Fixes: b6024b21fec8 ("um: extend fpstate to _xstate to support YMM registers")
> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
> ---
> Changes in v2:
> 
> - re-arrange the #ifdef/curly braces to not take the branch
> 
> arch/x86/um/os-Linux/registers.c | 12 ++++++++----
> arch/x86/um/user-offsets.c       |  4 ++++
> 2 files changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/um/os-Linux/registers.c b/arch/x86/um/os-Linux/registers.c
> index 00f54a91bb4b..28775f55bde2 100644
> --- a/arch/x86/um/os-Linux/registers.c
> +++ b/arch/x86/um/os-Linux/registers.c
> @@ -26,6 +26,7 @@ int save_i387_registers(int pid, unsigned long *fp_regs)
> 
> int save_fp_registers(int pid, unsigned long *fp_regs)
> {
> +#ifdef PTRACE_GETREGSET
>    struct iovec iov;
> 
>    if (have_xstate_support) {
> @@ -34,9 +35,9 @@ int save_fp_registers(int pid, unsigned long *fp_regs)
>        if (ptrace(PTRACE_GETREGSET, pid, NT_X86_XSTATE, &iov) < 0)
>            return -errno;
>        return 0;
> -    } else {
> +    } else
> +#endif
>        return save_i387_registers(pid, fp_regs);
> -    }
> }
> 
> int restore_i387_registers(int pid, unsigned long *fp_regs)
> @@ -48,6 +49,7 @@ int restore_i387_registers(int pid, unsigned long *fp_regs)
> 
> int restore_fp_registers(int pid, unsigned long *fp_regs)
> {
> +#ifdef PTRACE_SETREGSET
>    struct iovec iov;
> 
>    if (have_xstate_support) {
> @@ -56,9 +58,9 @@ int restore_fp_registers(int pid, unsigned long *fp_regs)
>        if (ptrace(PTRACE_SETREGSET, pid, NT_X86_XSTATE, &iov) < 0)
>            return -errno;
>        return 0;
> -    } else {
> +    } else
> +#endif
>        return restore_i387_registers(pid, fp_regs);
> -    }
> }
> 
> #ifdef __i386__
> @@ -122,6 +124,7 @@ int put_fp_registers(int pid, unsigned long *regs)
> 
> void arch_init_registers(int pid)
> {
> +#ifdef PTRACE_GETREGSET
>    struct _xstate fp_regs;
>    struct iovec iov;
> 
> @@ -129,6 +132,7 @@ void arch_init_registers(int pid)
>    iov.iov_len = sizeof(struct _xstate);
>    if (ptrace(PTRACE_GETREGSET, pid, NT_X86_XSTATE, &iov) == 0)
>        have_xstate_support = 1;

In my setup above call works correctly and so have_xstate_support will be set, but save_fp_registers fails with errno 4 or so...

> +#endif
> }
> #endif
> 
> diff --git a/arch/x86/um/user-offsets.c b/arch/x86/um/user-offsets.c
> index cb3c22370cf5..8af0fb5d2780 100644
> --- a/arch/x86/um/user-offsets.c
> +++ b/arch/x86/um/user-offsets.c
> @@ -50,7 +50,11 @@ void foo(void)
>    DEFINE(HOST_GS, GS);
>    DEFINE(HOST_ORIG_AX, ORIG_EAX);
> #else
> +#if defined(PTRACE_GETREGSET) && defined(PTRACE_SETREGSET)
>    DEFINE(HOST_FP_SIZE, sizeof(struct _xstate) / sizeof(unsigned long));
> +#else
> +    DEFINE(HOST_FP_SIZE, sizeof(struct _fpstate) / sizeof(unsigned long));
> +#endif
>    DEFINE_LONGS(HOST_BX, RBX);
>    DEFINE_LONGS(HOST_CX, RCX);
>    DEFINE_LONGS(HOST_DI, RDI);
> -- 
> 2.12.2
> 

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