Re: Save a WRMSR GS.base?

["H. Peter Anvin" <hpa@zytor.com>]

On 2026-06-08 07:38, Borislav Petkov wrote:
> On Sun, Jun 07, 2026 at 11:46:34PM -0700, H. Peter Anvin wrote:
>> WRxSBASE does a 64-bit write,
> 
> When REX.W.
> 
> The SDM text is confusing:
> 
> "If no REX.W prefix is used, the operand size is 32 bits; the upper 32 bits of
> the source register are ignored and upper 32 bits of the base address (for FS
> or GS) are cleared."
> 
> Does this last part that GS is cleared, refer to when WRGSBASE is used with no
> REX.W or in general?
> 

Without REX.W (e.g. wrgsbase %eax as opposed to wrgsbase %rax).

>> but for GS it would incorrectly address the kernel GS.base.
> 
> What does that mean?
> 

It means that in kernel mode, it is the currently active GS.base that is 
written (or read with rdgsbase), that is, the one that belongs to 
kernel, not the user space one in what is confusingly enough called 
MSR_KERNEL_GS_BASE.

In other words, not the one we want to task switch, *unless* you are in 
IDT mode and can surround it with SWAPGS.

	-hpa