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Message-ID: <aC0MIUOTQbb9-a7k@google.com>
Date: Tue, 20 May 2025 16:11:29 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.com>
Cc: "pbonzini@...hat.com" <pbonzini@...hat.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
"vipinsh@...gle.com" <vipinsh@...gle.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/3] KVM: x86: Use kvzalloc() to allocate VM struct
On Tue, May 20, 2025, Kai Huang wrote:
> On Mon, 2025-05-19 at 08:39 -0700, Sean Christopherson wrote:
> > +static int tdx_sept_remove_private_spte(struct kvm *kvm, gfn_t gfn,
> > + enum pg_level level, kvm_pfn_t pfn)
> > {
> > struct page *page = pfn_to_page(pfn);
> > int ret;
> > @@ -3507,10 +3507,14 @@ int __init tdx_bringup(void)
> > r = __tdx_bringup();
> > if (r) {
> > /*
> > - * Disable TDX only but don't fail to load module if
> > - * the TDX module could not be loaded. No need to print
> > - * message saying "module is not loaded" because it was
> > - * printed when the first SEAMCALL failed.
> > + * Disable TDX only but don't fail to load module if the TDX
> > + * module could not be loaded. No need to print message saying
> > + * "module is not loaded" because it was printed when the first
> > + * SEAMCALL failed. Don't bother unwinding the S-EPT hooks or
> > + * vm_size, as kvm_x86_ops have already been finalized (and are
> > + * intentionally not exported). The S-EPT code is unreachable,
> > + * and allocating a few more bytes per VM in a should-be-rare
> > + * failure scenario is a non-issue.
> > */
> > if (r == -ENODEV)
> > goto success_disable_tdx;
> > @@ -3524,3 +3528,19 @@ int __init tdx_bringup(void)
> > enable_tdx = 0;
> > return 0;
> > }
> > +
> > +
> > +void __init tdx_hardware_setup(void)
> > +{
> > + /*
> > + * Note, if the TDX module can't be loaded, KVM TDX support will be
> > + * disabled but KVM will continue loading (see tdx_bringup()).
> > + */
>
> This comment seems a little bit weird to me. I think what you meant here is the
> @vm_size and those S-EPT ops are not unwound while TDX cannot be brought up but
> KVM is still loaded.
This comment is weird? Or the one in tdx_bringup() is weird? The sole intent
of _this_ comment is to clarify that KVM could still end up running load with TDX
disabled. The comment about not unwinding S-EPT resides in tdx_bringup(), because
that's where the actual decision to not reject KVM load and to not undo the setup
lives.
> > +
> > + vt_x86_ops.link_external_spt = tdx_sept_link_private_spt;
> > + vt_x86_ops.set_external_spte = tdx_sept_set_private_spte;
> > + vt_x86_ops.free_external_spt = tdx_sept_free_private_spt;
> > + vt_x86_ops.remove_external_spte = tdx_sept_remove_private_spte;
> > + vt_x86_ops.protected_apic_has_interrupt = tdx_protected_apic_has_interrupt;
> > +}
> > diff --git a/arch/x86/kvm/vmx/tdx.h b/arch/x86/kvm/vmx/tdx.h
> > index 51f98443e8a2..ca39a9391db1 100644
> > --- a/arch/x86/kvm/vmx/tdx.h
> > +++ b/arch/x86/kvm/vmx/tdx.h
> > @@ -8,6 +8,7 @@
> > #ifdef CONFIG_KVM_INTEL_TDX
> > #include "common.h"
> >
> > +void tdx_hardware_setup(void);
> > int tdx_bringup(void);
> > void tdx_cleanup(void);
> >
>
> There's a build error when CONFIG_KVM_INTEL_TDX is off:
>
> vmx/main.c: In function ‘vt_hardware_setup’:
> vmx/main.c:34:17: error: implicit declaration of function ‘tdx_hardware_setup’;
> did you mean ‘vmx_hardware_setup’? [-Wimplicit-function-declaration]
> 34 | tdx_hardware_setup();
> | ^~~~~~~~~~~~~~~~~~
> | vmx_hardware_setup
>
> .. for which you need a stub for tdx_hardware_setup() when CONFIG_KVM_INTEL_TDX
> is off.
Not in kvm-x86/next, commit 907092bf7cbd ("KVM: VMX: Clean up and macrofy x86_ops")
buried all of vt_hardware_setup() behind CONFIG_KVM_INTEL_TDX=y.
> And one more thing:
>
> With the above patch, we still have below code in vt_init():
>
> /*
> * TDX and VMX have different vCPU structures. Calculate the
> * maximum size/align so that kvm_init() can use the larger
> * values to create the kmem_vcpu_cache.
> */
> vcpu_size = sizeof(struct vcpu_vmx);
> vcpu_align = __alignof__(struct vcpu_vmx);
> if (enable_tdx) {
> vcpu_size = max_t(unsigned, vcpu_size,
> sizeof(struct vcpu_tdx));
> vcpu_align = max_t(unsigned, vcpu_align,
> __alignof__(struct vcpu_tdx));
> kvm_caps.supported_vm_types |= BIT(KVM_X86_TDX_VM);
> }
>
> It's kinda ugly too IMHO.
>
> Since we already have @vm_size in kvm_x86_ops, how about also adding vcpu_size
> and vcpu_align to it? Then they can be treated in the same way as vm_size for
> TDX.
>
> They are not needed for SVM, but it doesn't hurt that much?
I'd rather not. vt_init() already needs to be aware of TDX, e.g. to call into
tdx_bringup() in the first place. Shoving state into kvm_x86_ops that is only
ever used in vt_init() (an __init function at that) isn't a net positive.
Putting the fields in kvm_x86_init_ops would be better, but I still don't think
the complexity and indirection is justified. Bleeding gory TDX details into the
common code is undesirable, but I don't see the size of vcpu_tdx or the fact that
enable_tdx==true means KVM_X86_TDX_VM is supported as being information with
hiding.
kvm_x86_ops.vm_size is a means to an end. E.g. if kvm_vcpu_cache didn't exist,
then we'd probably want/need kvm_x86_ops.vcpu_size, but it does exist, so...
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