Re: [PATCH] nestedsvm: Clear GIF when injecting VMEXIT

[Teddy Astie <teddy.astie@xxxxxxxxxx>]

Le 07/05/2026 à 15:03, Ross Lagerwall a écrit :
> On 4/29/26 5:54 PM, Teddy Astie wrote:
> > Le 23/04/2026 à 18:13, Ross Lagerwall a écrit :
> > > If L1 executes VMRUN with the GIF set and it fails consistency checks,
> > > Xen will inject a VMEXIT and fail the assert checking the GIF is 
> > > cleared.
> > > Instead, clear the GIF when injecting a VMEXIT to match what hardware
> > > does.
> > >
> > > Fixes: 9a779e4fc161 ("Implement SVM specific part for Nested 
> > > Virtualization")
> > > Signed-off-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> > > ---
> > >    xen/arch/x86/hvm/svm/nestedsvm.c | 4 ++--
> > >    1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/xen/arch/x86/hvm/svm/nestedsvm.c b/xen/arch/x86/hvm/svm/ 
> > > nestedsvm.c
> > > index ef6fa5d23b67..f89b087a1155 100644
> > > --- a/xen/arch/x86/hvm/svm/nestedsvm.c
> > > +++ b/xen/arch/x86/hvm/svm/nestedsvm.c
> > > @@ -733,9 +733,9 @@ nsvm_vcpu_vmexit_inject(struct vcpu *v, struct 
> > > cpu_user_regs *regs,
> > >        struct vmcb_struct *vmcb = v->arch.hvm.svm.vmcb;
> > >        if ( vmcb->_vintr.fields.vgif_enable )
> > > -        ASSERT(vmcb->_vintr.fields.vgif == 0);
> > > +        vmcb->_vintr.fields.vgif = 0;
> > >        else
> > > -        ASSERT(svm->ns_gif == 0);
> > > +        nestedsvm_vcpu_clgi(v);
> > >        ns_vmcb = nv->nv_vvmcx;
> > Looks good to me, though I think we are here looking to make a "guest
> > CLGI" (clear GIF), so the vGIF specific logic should be collapsed into
> > nestedsvm_vcpu_clgi() instead of having it as the non-vgif-support case.
> > (as IIUC, vGIF is a hardware accelration for nested GIF handling ?)
> Not 100% sure I follow your point here but v2 of this series removes
> nestedsvm_vcpu_clgi() entirely. Does that address your concern?
>
> > (also making me notice that svm_vmexit_do_{stgi,clgi}() seems to lack
> > vGIF specific logic)
> Isn't that by design? If vGIF is enabled the hardware should handle
> STGI/CLGI without a VMEXIT.
I think at some point in the future, we may need a 
`nestedsvm_vcpu_{stgi,clgi}()` function, or something in that regard; 
e.g for emulating this instruction outside of a SVM intercept path (x86 
emulator bits ?). As I think there are cases where the instruction is 
not intercepted but still needs to be emulated.
Such function would need to know whether or not we use vGIF.

> Ross
Teddy

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