Re: [kvm-unit-tests PATCH] x86: Initialize segment selectors

[Paolo Bonzini <pbonzini@redhat.com>]

On 24/06/20 14:52, Thomas Huth wrote:
> On 23/06/2020 10.41, Nadav Amit wrote:
>> Currently, the BSP's segment selectors are not initialized in 32-bit
>> (cstart.S). As a result the tests implicitly rely on the segment
>> selector values that are set by the BIOS. If this assumption is not
>> kept, the task-switch test fails.
>>
>> Fix it by initializing them.
>>
>> Signed-off-by: Nadav Amit <namit@vmware.com>
>> ---
>>   x86/cstart.S | 17 +++++++++++------
>>   1 file changed, 11 insertions(+), 6 deletions(-)
> 
> I'm sorry to be the bearer of bad news again, but this commit broke
> another set of tests in the Travis CI:
> 
>  https://travis-ci.com/github/huth/kvm-unit-tests/jobs/353103187#L796
> 
> smptest, smptest3, kvmclock_test, hyperv_synic and hyperv_stimer are
> failing now in the 32-bit kvm-unit-tests :-(

And that's just bad testing (both Nadav's and mine).  Writing to %gs
clobbers the PERCPU area.  The fix is as simple as this:

diff --git a/x86/cstart.S b/x86/cstart.S
index 5ad70b5..77dc34d 100644
--- a/x86/cstart.S
+++ b/x86/cstart.S
@@ -106,6 +106,7 @@ MSR_GS_BASE = 0xc0000101
 .globl start
 start:
         mov $stacktop, %esp
+        setup_segments
         push %ebx
         call setup_multiboot
         call setup_libcflat
@@ -118,7 +119,6 @@ start:

 prepare_32:
         lgdtl gdt32_descr
-	setup_segments

 	mov %cr4, %eax
 	bts $4, %eax  // pse