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    Subject[PATCH 3/3] xen/pv: support selecting safe/unsafe msr accesses
    Date
    Instead of always doing the safe variants for reading and writing MSRs
    in Xen PV guests, make the behavior controllable via Kconfig option
    and a boot parameter.

    The default will be the current behavior, which is to always use the
    safe variant.

    Signed-off-by: Juergen Gross <jgross@suse.com>
    ---
    .../admin-guide/kernel-parameters.txt | 6 +++++
    arch/x86/xen/Kconfig | 9 +++++++
    arch/x86/xen/enlighten_pv.c | 24 +++++++++++--------
    3 files changed, 29 insertions(+), 10 deletions(-)

    diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
    index 426fa892d311..1bda9cf18fae 100644
    --- a/Documentation/admin-guide/kernel-parameters.txt
    +++ b/Documentation/admin-guide/kernel-parameters.txt
    @@ -6836,6 +6836,12 @@
    Crash from Xen panic notifier, without executing late
    panic() code such as dumping handler.

    + xen_msr_safe= [X86,XEN]
    + Format: <bool>
    + Select whether to always use non-faulting (safe) MSR
    + access functions when running as Xen PV guest. The
    + default value is controlled by CONFIG_XEN_PV_MSR_SAFE.
    +
    xen_nopvspin [X86,XEN]
    Disables the qspinlock slowpath using Xen PV optimizations.
    This parameter is obsoleted by "nopvspin" parameter, which
    diff --git a/arch/x86/xen/Kconfig b/arch/x86/xen/Kconfig
    index 85246dd9faa1..9b1ec5d8c99c 100644
    --- a/arch/x86/xen/Kconfig
    +++ b/arch/x86/xen/Kconfig
    @@ -92,3 +92,12 @@ config XEN_DOM0
    select X86_X2APIC if XEN_PVH && X86_64
    help
    Support running as a Xen Dom0 guest.
    +
    +config XEN_PV_MSR_SAFE
    + bool "Always use safe MSR accesses in PV guests"
    + default y
    + depends on XEN_PV
    + help
    + Use safe (not faulting) MSR access functions even if the MSR access
    + should not fault anyway.
    + The default can be changed by using the "xen_msr_safe" boot parameter.
    diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
    index 4e68e047df94..6b0e5d4c485a 100644
    --- a/arch/x86/xen/enlighten_pv.c
    +++ b/arch/x86/xen/enlighten_pv.c
    @@ -108,6 +108,16 @@ struct tls_descs {
    */
    static DEFINE_PER_CPU(struct tls_descs, shadow_tls_desc);

    +static __read_mostly bool xen_msr_safe = IS_ENABLED(CONFIG_XEN_PV_MSR_SAFE);
    +
    +static int __init parse_xen_msr_safe(char *str)
    +{
    + if (str)
    + return strtobool(str, &xen_msr_safe);
    + return -EINVAL;
    +}
    +early_param("xen_msr_safe", parse_xen_msr_safe);
    +
    static void __init xen_pv_init_platform(void)
    {
    /* PV guests can't operate virtio devices without grants. */
    @@ -1010,22 +1020,16 @@ static int xen_write_msr_safe(unsigned int msr, unsigned int low,

    static u64 xen_read_msr(unsigned int msr)
    {
    - /*
    - * This will silently swallow a #GP from RDMSR. It may be worth
    - * changing that.
    - */
    int err;

    - return xen_read_msr_safe(msr, &err);
    + return xen_do_read_msr(msr, xen_msr_safe ? &err : NULL);
    }

    static void xen_write_msr(unsigned int msr, unsigned low, unsigned high)
    {
    - /*
    - * This will silently swallow a #GP from WRMSR. It may be worth
    - * changing that.
    - */
    - xen_write_msr_safe(msr, low, high);
    + int err;
    +
    + xen_do_write_msr(msr, low, high, xen_msr_safe ? &err : NULL);
    }

    /* This is called once we have the cpu_possible_mask */
    --
    2.35.3
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