Xen devel - Sep 2009 - cygwin longjmp error on Xen guest - gs not restored?

[resending to xen-devel instead of xen-users as suggested]

Hi All

A colleague tells me there is a problem with Xen 3.0.3 on RHEL5 where
Cygwin''s bash fails to call longjmp() in a Windows guest.

The consensus is that Xen and KVM both had a bug, but that Xen has not
yet fixed it.  Indeed I couldn''t find any proof that it had been
reported to Xen, hence this email.

Original KVM bug report:
http://sourceforge.net/tracker/?func=detail&atid=893831&aid=1872255&group_id=180599

Test case:
http://article.gmane.org/gmane.comp.emulators.kvm.devel/28099

#include <setjmp.h>

  jmp_buf env;
  main()
  {
       if(setjmp(env)) return;
       longjmp(env, 1);
  }

More details: it''s something to do with the GS register:
http://markmail.org/message/5aqlqtrld4ti4gse

#include <stdio.h>

int main()
{
  unsigned short gs;
  unsigned x;

  asm ("mov %%gs, %0\n" : "=g"(gs));
  asm ("movl %%gs:0x30, %0\n" : "=r"(x));

  printf("gs: %x\n", gs);
  printf("gs:0x30: %x\n", x);

  asm ("mov %0, %%gs\n" : : "g"(gs));

  printf("test\n");

  asm ("movl %%gs:0x30, %0\n" : "=r"(x));

  return 0;
}


KVM fix:
http://article.gmane.org/gmane.comp.emulators.kvm.devel/28418

diff --git a/kernel/x86/vmx.c b/kernel/x86/vmx.c
index 7507ce2..048460d 100644
--- a/kernel/x86/vmx.c
+++ b/kernel/x86/vmx.c
@@ -910,6 +910,7 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, u32 msr_index,
u64 *pdata)
 		data = vmcs_readl(GUEST_SYSENTER_ESP);
 		break;
 	default:
+		vmx_load_host_state(vcpu);
 		msr = find_msr_entry(to_vmx(vcpu), msr_index);
 		if (msr) {
 			data = msr->data;


Another related KVM thread:
http://markmail.org/message/owy3x7pf6oywdx5e

Cygwin developers:
http://www.cygwin.com/ml/cygwin/2009-05/msg00841.html
http://www.cygwin.com/ml/cygwin/2008-08/msg00437.html


I''ll be happy to spend some time helping to test/diagnose if you can
tell me what you need me to do.

Thanks

Mike



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On 16/09/2009 10:59, "Mikel Ward" <mward@aconex.com> wrote:
> [resending to xen-devel instead of xen-users as suggested]
> 
> Hi All
> 
> A colleague tells me there is a problem with Xen 3.0.3 on RHEL5 where
> Cygwin''s bash fails to call longjmp() in a Windows guest.
Have you tested this with a newer version of Xen than 3.0.3? For example the
latest stable release, 3.4.1? In all likelihood this is already fixed.

 -- Keir



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