[PATCH v2] KVM: PPC: Align pt_regs in kvm_vcpu_arch structure

[PATCH v2] KVM: PPC: Align pt_regs in kvm_vcpu_arch structure

[Fabiano Rosas]

The H_ENTER_NESTED hypercall receives as second parameter the address
of a region of memory containing the values for the nested guest
privileged registers. We currently use the pt_regs structure contained
within kvm_vcpu_arch for that end.

Most hypercalls that receive a memory address expect that region to
not cross a 4K page boundary. We would want H_ENTER_NESTED to follow
the same pattern so this patch ensures the pt_regs structure sits
within a page.

Note: the pt_regs structure is currently 384 bytes in size, so
aligning to 512 is sufficient to ensure it will not cross a 4K page
and avoids punching too big a hole in struct kvm_vcpu_arch.

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Signed-off-by: Murilo Opsfelder Araújo <muriloo@linux.ibm.com>
---
v2:
 - updated commit message to inform the rationale for aligning to 512;

 - added Murilo's sign-off which I had forgotten, we worked on this
   together.
---
 arch/powerpc/include/asm/kvm_host.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/kvm_host.h b/arch/powerpc/include/asm/kvm_host.h
index 2909a88acd16..2c7219cef4ec 100644
--- a/arch/powerpc/include/asm/kvm_host.h
+++ b/arch/powerpc/include/asm/kvm_host.h
@@ -523,7 +523,11 @@ struct kvm_vcpu_arch {
 	struct kvmppc_book3s_shadow_vcpu *shadow_vcpu;
 #endif
 
-	struct pt_regs regs;
+	/*
+	 * This is passed along to the HV via H_ENTER_NESTED. Align to
+	 * prevent it crossing a real 4K page.
+	 */
+	struct pt_regs regs __aligned(512);
 
 	struct thread_fp_state fp;
 
-- 
2.35.3

Re: [PATCH v2] KVM: PPC: Align pt_regs in kvm_vcpu_arch structure

[Michael Ellerman]

On Fri, 24 Jun 2022 11:27:12 -0300, Fabiano Rosas wrote:
> The H_ENTER_NESTED hypercall receives as second parameter the address
> of a region of memory containing the values for the nested guest
> privileged registers. We currently use the pt_regs structure contained
> within kvm_vcpu_arch for that end.
> 
> Most hypercalls that receive a memory address expect that region to
> not cross a 4K page boundary. We would want H_ENTER_NESTED to follow
> the same pattern so this patch ensures the pt_regs structure sits
> within a page.
> 
> [...]

Applied to powerpc/topic/ppc-kvm.

[1/1] KVM: PPC: Align pt_regs in kvm_vcpu_arch structure
      https://git.kernel.org/powerpc/c/f5c847ea19d323974d6f7c7e9fa4858ce0727096

cheers