Re: [Xen-devel] [PATCH 02/11] x86/vmx: Don't clobber %dr6 while debugging state is lazy

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Jun 04, 2018 at 02:59:06PM +0100, Andrew Cooper wrote:
> c/s 4f36452b63 introduced a write to %dr6 in the #DB intercept case, but the
> guests debug registers may be lazy at this point, at which point the guests
> later attempt to read %dr6 will discard this value and use the older stale
> value.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Jun Nakajima <jun.nakajima@xxxxxxxxx>
> CC: Kevin Tian <kevin.tian@xxxxxxxxx>
> ---
>  xen/arch/x86/hvm/vmx/vmx.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> index 33d39f6..8dbe838 100644
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -3696,6 +3696,7 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
>               */
>              __vmread(EXIT_QUALIFICATION, &exit_qualification);
>              HVMTRACE_1D(TRAP_DEBUG, exit_qualification);
> +            __restore_debug_registers(v);

If I understood this correctly, you only call
__restore_debug_registers in order to make sure the DR registers are
marked as dirty?

Thanks, Roger.

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