Re: [PATCH] hvf: Fix segment selector format

[Paolo Bonzini]

On 16/11/20 21:04, Jessica Clarke wrote:
> The Requested Privilege Level field is 2 bits, the Table Indicator field
> is 1 bit and the Index field is the remaining 15 bits, with TI=0 meaning
> GDT and TI=1 meaning LDT.
>
> Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com>
> ---
>   target/i386/hvf/x86.h | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/target/i386/hvf/x86.h b/target/i386/hvf/x86.h
> index bacade7b65..ea3e1b86b3 100644
> --- a/target/i386/hvf/x86.h
> +++ b/target/i386/hvf/x86.h
> @@ -214,16 +214,16 @@ static inline uint32_t x86_call_gate_offset(x86_call_gate 
> *gate)
>       return (uint32_t)((gate->offset1 << 16) | gate->offset0);
>   }
>   
> -#define LDT_SEL     0
> -#define GDT_SEL     1
> +#define GDT_SEL     0
> +#define LDT_SEL     1
>   
>   typedef struct x68_segment_selector {
>       union {
>           uint16_t sel;
>           struct {
> -            uint16_t rpl:3;
> +            uint16_t rpl:2;
>               uint16_t ti:1;
> -            uint16_t index:12;
> +            uint16_t index:13;
>           };
>       };
>   } __attribute__ ((__packed__)) x68_segment_selector;

I queued the patch, thanks.

On further look, though, the bitfield part of the struct is almost never 
used, and therefore most uses of the struct itself are more or less 
superfluous (apart from some typechecking).  In particular, 
vmx_read_segment_selector and vmx_write_segment_selector only use the 
16-bit .self field, and the code would be simpler if it was changed to 
just use a uint16_t.

The only place that "needs" the struct is in vmx_handle_task_switch's 
calls to x86_read_segment_descriptor and x86_write_segment_descriptor. 
Those are also the places that benefit from this patch.  But even then, 
for the sake of consistency it would make sense for x86_segment_selector 
to be used only inside those two functions; the arguments could be just 
an uint16_t.

Paolo