Re: [kvm-unit-tests PATCH 01/14] x86/cstart: Don't use MSR_GS_BASE in 32-bit boot code

Paolo Bonzini
2021-04-22T09:44:36Z

On 22/04/21 05:04, Sean Christopherson wrote:
> Load the per-cpu GS.base for 32-bit build by building a temporary GDT
> and loading a "real" segment. Using MSR_GS_BASE is wrong and broken,
> it's a 64-bit only MSR and does not exist on 32-bit CPUs. The current
> code works only because 32-bit KVM VMX incorrectly disables interception
> of MSR_GS_BASE, and no one runs KVM on an actual 32-bit physical CPU,
> i.e. the MSR exists in hardware and so everything "works".
>
> 32-bit KVM SVM is not buggy and correctly injects #GP on the WRMSR, i.e.
> the tests have never worked on 32-bit SVM.
>
> Fixes: dfe6cb6 ("Add 32 bit smp initialization code")
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Relying on the descriptor cache is quite ugly but the only alternative
are setting up extra segments in the GDT or having per-CPU GDTs (which
I'd rather avoid).

Paolo

> ---
> x86/cstart.S | 28 +++++++++++++++++++++++-----
> 1 file changed, 23 insertions(+), 5 deletions(-)
>
> diff --git a/x86/cstart.S b/x86/cstart.S
> index 489c561..91970a2 100644
> --- a/x86/cstart.S
> +++ b/x86/cstart.S
> @@ -89,13 +89,31 @@ mb_flags = 0x0
> .long mb_magic, mb_flags, 0 - (mb_magic + mb_flags)
> mb_cmdline = 16
>
> -MSR_GS_BASE = 0xc0000101
> -
> .macro setup_percpu_area
> lea -4096(%esp), %eax
> - mov $0, %edx
> - mov $MSR_GS_BASE, %ecx
> - wrmsr
> +
> + mov %eax, %edx
> + shl $16, %edx
> + or $0xffff, %edx
> + mov %edx, 0x10(%eax)
> +
> + mov %eax, %edx
> + and $0xff000000, %edx
> + mov %eax, %ecx
> + shr $16, %ecx
> + and $0xff, %ecx
> + or %ecx, %edx
> + or $0x00cf9300, %edx
> + mov %edx, 0x14(%eax)
> +
> + movw $0x17, 0(%eax)
> + mov %eax, 2(%eax)
> + lgdtl (%eax)
> +
> + mov $0x10, %ax
> + mov %ax, %gs
> +
> + lgdtl gdt32_descr
> .endm
>
> .macro setup_segments
>