[Xen-devel] [patch 3/3] VT-d cleanup: correct way to submit command to GCMD register

["Han, Weidong" <weidong.han@xxxxxxxxx>]

Per VT-d spec, software should submit only one "incremental" command at a time 
to Global Command reigster. Current implementation uses a variable (gcmd) to 
record the state of Global Status register. It's error prone. A good algorithm 
to use is as follows, whenver software needs to submit a command:
        Read current value of Global Status Register (X);               //  
This tells what is the current state of commands enabled in hardware
        X = Bitwise OR the new command bit to X;                // The only new 
bit set in X is the one new command we are going to issue
        Write X  to Global Command register;                    // Submit the 
command
        Poll Global Status register until it has new value of X ;       // Poll 
until command is done


Signed-off-by: Weidong Han <weidong.han@xxxxxxxxx>

Attachment: gcmd.patch
Description: gcmd.patch

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