Re: [Xen-devel] [PATCH 1/2] x86/vmx: Don't clobber exception_bitmap when entering/leaving emulated real mode

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> Sent: Thursday, January 28, 2016 2:11 AM
> 
> Most updates to the exception bitmaps set or clear an individual bits.
> 
> However, entering or exiting emulated real mode unilaterally clobbers it,
> leaving the exit code to recalculate what it should have been.  This is error
> prone, and indeed currently fails to recalculate the TRAP_no_device intercept
> appropriately.
> 
> Instead of overwriting exception_bitmap when entering emulated real mode, move
> the override into vmx_update_exception_bitmap() and leave exception_bitmap
> unmodified.
> 
> This means that recalculation is unnecessary, and that the use of
> vmx_fpu_leave() and vmx_update_debug_state() while in emulated real mode
> doesn't result in TRAP_no_device and TRAP_int3 being un-intercepted.
> 
> This is only a functional change on hardware lacking unrestricted guest
> support.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Acked-by: Kevin Tian <kevin.tian@xxxxxxxxx>

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