Re: [PATCH v19 069/130] KVM: TDX: Require TDP MMU and mmio caching for TDX

Sean Christopherson
2024-04-01T17:34:07Z

On Mon, Feb 26, 2024, isaku.yamahata@intel.com wrote:
> From: Isaku Yamahata <isaku.yamahata@intel.com>
>
> As TDP MMU is becoming main stream than the legacy MMU, the legacy MMU
> support for TDX isn't implemented. TDX requires KVM mmio caching. Disable
> TDX support when TDP MMU or mmio caching aren't supported.
>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> ---
> arch/x86/kvm/mmu/mmu.c | 1 +
> arch/x86/kvm/vmx/main.c | 13 +++++++++++++
> 2 files changed, 14 insertions(+)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 0e0321ad9ca2..b8d6ce02e66d 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -104,6 +104,7 @@ module_param_named(flush_on_reuse, force_flush_and_sync_on_reuse, bool, 0644);
> * If the hardware supports that we don't need to do shadow paging.
> */
> bool tdp_enabled = false;
> +EXPORT_SYMBOL_GPL(tdp_enabled);

I haven't looked at the rest of the series, but this should be unnecessary. Just
use enable_ept. Ah, the code is wrong.

> static bool __ro_after_init tdp_mmu_allowed;
>
> diff --git a/arch/x86/kvm/vmx/main.c b/arch/x86/kvm/vmx/main.c
> index 076a471d9aea..54df6653193e 100644
> --- a/arch/x86/kvm/vmx/main.c
> +++ b/arch/x86/kvm/vmx/main.c
> @@ -3,6 +3,7 @@
>
> #include "x86_ops.h"
> #include "vmx.h"
> +#include "mmu.h"
> #include "nested.h"
> #include "pmu.h"
> #include "tdx.h"
> @@ -36,6 +37,18 @@ static __init int vt_hardware_setup(void)
> if (ret)
> return ret;
>
> + /* TDX requires KVM TDP MMU. */

This is a useless comment (assuming the code is correctly written), it's quite
obvious from:

if (!tdp_mmu_enabled)
enable_tdx = false;

that the TDP MMU is required. Explaining *why* could be useful, but I'd probably
just omit a comment. In the not too distant future, it will likely be common
knowledge that the shadow MMU doesn't support newfangled features, and it _should_
be very easy for someone to get the info from the changelog.

> + if (enable_tdx && !tdp_enabled) {

tdp_enabled can be true without the TDP MMU, you want tdp_mmu_enabled.

> + enable_tdx = false;
> + pr_warn_ratelimited("TDX requires TDP MMU. Please enable TDP MMU for TDX.\n");

Drop the pr_warn(), TDX will presumably be on by default at some point, I don't
want to get spam every time I test with TDP disabled.

Also, ratelimiting this code is pointless (as is _once()), it should only ever
be called once per module module, and the limiting/once protections are tied to
the module, i.e. effectively get reset when a module is reloaded.

> + }
> +
> + /* TDX requires MMIO caching. */
> + if (enable_tdx && !enable_mmio_caching) {
> + enable_tdx = false;
> + pr_warn_ratelimited("TDX requires mmio caching. Please enable mmio caching for TDX.\n");

Same comments here.

> + }
> +
> enable_tdx = enable_tdx && !tdx_hardware_setup(&vt_x86_ops);

All of the above code belongs in tdx_hardware_setup(), especially since you need
tdp_mmu_enabled, not enable_ept.

> if (enable_tdx)
> vt_x86_ops.vm_size = max_t(unsigned int, vt_x86_ops.vm_size,
> --
> 2.25.1
>