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    Subject[PATCH RFC] uaccess: user_access_begin_after_access_ok()
    So vhost needs to poke at userspace *a lot* in a quick succession.  It
    is thus benefitial to enable userspace access, do our thing, then
    disable. Except access_ok has already been pre-validated with all the
    relevant nospec checks, so we don't need that. Add an API to allow
    userspace access after access_ok and barrier_nospec are done.

    Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
    ---

    Jason, so I've been thinking using something along these lines,
    then switching vhost to use unsafe_copy_to_user and friends would
    solve lots of problems you observed with SMAP.

    What do you think? Do we need any other APIs to make it practical?

    arch/x86/include/asm/uaccess.h | 1 +
    include/linux/uaccess.h | 1 +
    2 files changed, 2 insertions(+)

    diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
    index d8f283b9a569..fa5afb3a54fe 100644
    --- a/arch/x86/include/asm/uaccess.h
    +++ b/arch/x86/include/asm/uaccess.h
    @@ -483,6 +483,7 @@ static __must_check __always_inline bool user_access_begin(const void __user *pt
    return 1;
    }
    #define user_access_begin(a,b) user_access_begin(a,b)
    +#define user_access_begin_after_access_ok() __uaccess_begin()
    #define user_access_end() __uaccess_end()

    #define user_access_save() smap_save()
    diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
    index 67f016010aad..4c0a959ad639 100644
    --- a/include/linux/uaccess.h
    +++ b/include/linux/uaccess.h
    @@ -370,6 +370,7 @@ extern long strnlen_unsafe_user(const void __user *unsafe_addr, long count);

    #ifndef user_access_begin
    #define user_access_begin(ptr,len) access_ok(ptr, len)
    +#define user_access_begin_after_access_ok() do { } while (0)
    #define user_access_end() do { } while (0)
    #define unsafe_op_wrap(op, err) do { if (unlikely(op)) goto err; } while (0)
    #define unsafe_get_user(x,p,e) unsafe_op_wrap(__get_user(x,p),e)
    --
    MST
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