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

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Message-ID: <20240402060357.GV2444378@ls.amr.corp.intel.com>
Date: Mon, 1 Apr 2024 23:03:57 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: isaku.yamahata@...el.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
	Paolo Bonzini <pbonzini@...hat.com>, erdemaktas@...gle.com,
	Sagi Shahar <sagis@...gle.com>, Kai Huang <kai.huang@...el.com>,
	chen.bo@...el.com, hang.yuan@...el.com, tina.zhang@...el.com,
	isaku.yamahata@...ux.intel.com
Subject: Re: [PATCH v19 069/130] KVM: TDX: Require TDP MMU and mmio caching
 for TDX

On Mon, Apr 01, 2024 at 10:34:05AM -0700,
Sean Christopherson <seanjc@...gle.com> wrote:

> On Mon, Feb 26, 2024, isaku.yamahata@...el.com wrote:
> > From: Isaku Yamahata <isaku.yamahata@...el.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@...el.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.

Thanks for review.  With tdp_mmu_enabled, removing warning and comments,
I come up with the followings.

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index aa66b0510062..ba2738cc6e98 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -110,6 +110,7 @@ static bool __ro_after_init tdp_mmu_allowed;
 #ifdef CONFIG_X86_64
 bool __read_mostly tdp_mmu_enabled = true;
 module_param_named(tdp_mmu, tdp_mmu_enabled, bool, 0444);
+EXPORT_SYMBOL_GPL(tdp_mmu_enabled);
 #endif
 
 static int max_huge_page_level __read_mostly;
diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 976cf5e37f0f..e6f66f7c04bb 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -1253,14 +1253,9 @@ int __init tdx_hardware_setup(struct kvm_x86_ops *x86_ops)
 	int r = 0;
 	int i;
 
-	if (!cpu_feature_enabled(X86_FEATURE_MOVDIR64B)) {
-		pr_warn("MOVDIR64B is reqiured for TDX\n");
+	if (!tdp_mmu_enabled || !enable_mmio_caching ||
+	    !cpu_feature_enabled(X86_FEATURE_MOVDIR64B))
 		return -EOPNOTSUPP;
-	}
-	if (!enable_ept) {
-		pr_warn("Cannot enable TDX with EPT disabled\n");
-		return -EINVAL;
-	}
 
 	max_pkgs = topology_max_packages();
 	tdx_mng_key_config_lock = kcalloc(max_pkgs, sizeof(*tdx_mng_key_config_lock),
-- 
2.43.2

-- 
Isaku Yamahata <isaku.yamahata@...el.com>