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

Hi, Fabiano.

On 5/25/22 09:49, 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.
> 
> Signed-off-by: Fabiano Rosas <[email protected]>

Is it necessary to explain in the commit message that even though the second
parameter needs to be 4k-aligned, we're aligning pt_regs to 512 bytes so it can
be placed within a 4k boundary because its size is below 512 bytes?

The natural thinking would be aligning it to 4k bytes, which would punch a huge
hole in kvm_vcpu_arch. I think having the explanation of why 512 vs. 4k is
worthwhile mentioning.

> ---
>   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 faf301d0dec0..87eba60f2920 100644
> --- a/arch/powerpc/include/asm/kvm_host.h
> +++ b/arch/powerpc/include/asm/kvm_host.h
> @@ -519,7 +519,11 @@ 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;
>   


-- 
Murilo