Re: [PATCH] KVM: x86: obey KVM_X86_QUIRK_CD_NW_CLEARED in kvm_set_cr0()

[Laszlo Ersek <lersek@redhat.com>]

On 11/03/15 19:34, Laszlo Ersek wrote:
> Commit b18d5431acc7 ("KVM: x86: fix CR0.CD virtualization") was
> technically correct, but it broke OVMF guests by slowing down various
> parts of the firmware.
> 
> Commit fb279950ba02 ("KVM: vmx: obey KVM_QUIRK_CD_NW_CLEARED") quirked the
> first function modified by b18d5431acc7, vmx_get_mt_mask(), for OVMF's
> sake. This restored the speed of the OVMF code that runs before
> PlatformPei (including the memory intensive LZMA decompression in SEC).
> 
> This patch extends the quirk to the second function modified by
> b18d5431acc7, kvm_set_cr0(). It eliminates the intrusive slowdown that
> hits the EFI_MP_SERVICES_PROTOCOL implementation of edk2's
> UefiCpuPkg/CpuDxe -- which is built into OVMF --, when CpuDxe starts up
> all APs at once for initialization, in order to count them.
> 
> We also carry over the kvm_arch_has_noncoherent_dma() sub-condition from
> the other half of the original commit b18d5431acc7.
> 
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Cc: Janusz Mocek <januszmk6@gmail.com>
> Cc: Alex Williamson <alex.williamson@redhat.com>
> Cc: Xiao Guangrong <guangrong.xiao@linux.intel.com>
> Signed-off-by: Laszlo Ersek <lersek@redhat.com>
> ---
>  arch/x86/kvm/x86.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index a24bae0..30723a4 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -625,7 +625,9 @@ int kvm_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0)
>  	if ((cr0 ^ old_cr0) & update_bits)
>  		kvm_mmu_reset_context(vcpu);
>  
> -	if ((cr0 ^ old_cr0) & X86_CR0_CD)
> +	if (((cr0 ^ old_cr0) & X86_CR0_CD) &&
> +	    kvm_arch_has_noncoherent_dma(vcpu->kvm) &&
> +	    !kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_CD_NW_CLEARED))
>  		kvm_zap_gfn_range(vcpu->kvm, 0, ~0ULL);
>  
>  	return 0;
> 


I had notes on this patch, but I forgot to format it with --notes. They
were:

- People on the CC list, please reply with your Tested-by, Reported-by,
  etc tags as appropriate; it's getting blurry who participated in what
  and how.

- This patch is *not* necessary for the OVMF SMM work; instead it
  addresses an independent OVMF boot regression seen by users.

Thanks
Laszlo