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Re: [PATCH v2] x86/svm: Intercept CR0 writes selectively
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>
- From: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
- Date: Tue, 8 Sep 2026 15:09:12 +0100
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- Cc: Roger Pau Monné <roger@xxxxxxxxxxxxxx>, Jason Andryuk <jason.andryuk@xxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Tue, 08 Sep 2026 14:09:44 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 9/8/26 2:57 PM, Andrew Cooper wrote:
On 08/09/2026 9:44 am, Jan Beulich wrote:
On 04.09.2026 14:23, Ross Lagerwall wrote:
@@ -2517,6 +2518,7 @@ void asmlinkage svm_vmexit_handler(void)
hvm_sanitize_regs_fields(
regs, !(vmcb_get_efer(vmcb) & EFER_LMA) || !(vmcb->cs.l));
+ v->arch.hvm.guest_cr[0] = vmcb_get_cr0(vmcb);
v->arch.hvm.guest_cr[2] = vmcb_get_cr2(vmcb);
if ( paging_mode_hap(v->domain) )
v->arch.hvm.guest_cr[3] = v->arch.hvm.hw_cr[3] = vmcb_get_cr3(vmcb);
gitlab-CI says no to this patch [1], and I think it's the change above which
gets in the way of running guests in shadow mode (as is the case for the two
qemu-smoke-x86_64-*-pvh tests). svm_update_guest_cr() has
value = v->arch.hvm.guest_cr[0];
if ( paging_mode_shadow(v->domain) )
value |= X86_CR0_PG | X86_CR0_WP;
vmcb_set_cr0(vmcb, value);
i.e. upon reading back the guest's CR0 value we would wrongly record it
having PG (and WP) enabled, at which point shadow code would try to use the
still-zero CR3 as page table base. I think values need merging here, with
only TS and MP taken from what vmcb_get_cr0() returns.
Yes this is broken for Shadow.
Shadow shouldn't be trying to use CR0_WRITE_SEL. What's in the VMCB is
unrelated to the guest's choice of value, and reverse engineering it is
fragile.
Use full CR0_WRITE for Shadow guests, and use CR0_WRITE_SEL for HAP only.
I did try Jan's suggestion which fixes the initial problem but crashes when
dom0 brings up its second vCPU.
If keeping Shadow guests using full CR0_WRITE is acceptable, I'll update the
patch to do that since there are surely better things to do than spending time
making Shadow improvements.
Ross
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