Re: [PATCH V5 4/6] x86/hvm: pkeys, add pkeys support for guest_walk_tables

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 22/12/15 10:30, Huaitong Han wrote:
> --- a/xen/include/asm-x86/processor.h
> +++ b/xen/include/asm-x86/processor.h
> @@ -373,6 +373,45 @@ static always_inline void clear_in_cr4 (unsigned long mask)
>      write_cr4(read_cr4() & ~mask);
>  }
>  
> +static inline unsigned int read_pkru(void)
> +{
> +    unsigned int pkru;
> +
> +    /*
> +     * _PAGE_PKEY_BITS have a conflict with _PAGE_GNTTAB used by PV guests,
> +     * so that X86_CR4_PKE is disable on hypervisor, RDPKRU instruction can
> +     * be used with temporarily setting CR4.PKE.
> +     */
> +    set_in_cr4(X86_CR4_PKE);
> +    asm volatile (".byte 0x0f,0x01,0xee"
> +        : "=a" (pkru) : "c" (0) : "dx");
> +    clear_in_cr4(X86_CR4_PKE);

You can't use set/clear_in_cr4 here, as it modifies the global
mmu_cr4_features variable.

I would recommend

unsigned long cr4 = read_cr4();
write_cr4(cr4 | X86_CR4_PKE);
...
write_cr4(cr4);

instead.