linux-kernel - Re: [PATCH v4 3/7] x86/virt/tdx: Mark memory cache state incoherent when making SEAMCALL

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Message-ID: <aH+lx0vJE5KA7ifd@intel.com>
Date: Tue, 22 Jul 2025 22:52:55 +0800
From: Chao Gao <chao.gao@...el.com>
To: Kai Huang <kai.huang@...el.com>
CC: <dave.hansen@...el.com>, <bp@...en8.de>, <tglx@...utronix.de>,
	<peterz@...radead.org>, <mingo@...hat.com>, <hpa@...or.com>,
	<thomas.lendacky@....com>, <x86@...nel.org>, <kas@...nel.org>,
	<rick.p.edgecombe@...el.com>, <dwmw@...zon.co.uk>,
	<linux-kernel@...r.kernel.org>, <pbonzini@...hat.com>, <seanjc@...gle.com>,
	<kvm@...r.kernel.org>, <reinette.chatre@...el.com>,
	<isaku.yamahata@...el.com>, <dan.j.williams@...el.com>,
	<ashish.kalra@....com>, <nik.borisov@...e.com>, <sagis@...gle.com>, "Farrah
 Chen" <farrah.chen@...el.com>
Subject: Re: [PATCH v4 3/7] x86/virt/tdx: Mark memory cache state incoherent
 when making SEAMCALL

>+static __always_inline u64 do_seamcall(sc_func_t func, u64 fn,
>+				       struct tdx_module_args *args)
>+{
>+	u64 ret;
>+
>+	lockdep_assert_preemption_disabled();
>+
>+	/*
>+	 * SEAMCALLs are made to the TDX module and can generate dirty
>+	 * cachelines of TDX private memory.  Mark cache state incoherent
>+	 * so that the cache can be flushed during kexec.
>+	 *
>+	 * This needs to be done before actually making the SEAMCALL,
>+	 * because kexec-ing CPU could send NMI to stop remote CPUs,
>+	 * in which case even disabling IRQ won't help here.
>+	 */
>+	this_cpu_write(cache_state_incoherent, true);
>+
>+	ret = func(fn, args);
>+
>+	return ret;

@ret can be dropped here. Just

	return func(fn, args);

should work.

And tracking cache incoherent state at the per-CPU level seems to add
unnecessary complexity. It requires a new do_seamcall() wrapper, setting the
flag on every seamcall rather than just the first one (I'm not concerned about
performance; it just feels silly), and using preempt_disable()/enable(). In my
view, per-CPU tracking at most saves a WBINVD on a CPU that never runs
SEAMCALLs during KEXEC, which is quite marginal. Did I miss any other benefits?

>+}
>+
> static __always_inline u64 sc_retry(sc_func_t func, u64 fn,
> 			   struct tdx_module_args *args)
> {
>@@ -113,7 +138,9 @@ static __always_inline u64 sc_retry(sc_func_t func, u64 fn,
> 	u64 ret;
> 
> 	do {
>-		ret = func(fn, args);
>+		preempt_disable();
>+		ret = do_seamcall(func, fn, args);
>+		preempt_enable();
> 	} while (ret == TDX_RND_NO_ENTROPY && --retry);
> 
> 	return ret;
>diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
>index c7a9a087ccaf..d6ee4e5a75d2 100644
>--- a/arch/x86/virt/vmx/tdx/tdx.c
>+++ b/arch/x86/virt/vmx/tdx/tdx.c
>@@ -1266,7 +1266,7 @@ static bool paddr_is_tdx_private(unsigned long phys)
> 		return false;
> 
> 	/* Get page type from the TDX module */
>-	sret = __seamcall_ret(TDH_PHYMEM_PAGE_RDMD, &args);
>+	sret = do_seamcall(__seamcall_ret, TDH_PHYMEM_PAGE_RDMD, &args);
> 
> 	/*
> 	 * The SEAMCALL will not return success unless there is a
>@@ -1522,7 +1522,7 @@ noinstr __flatten u64 tdh_vp_enter(struct tdx_vp *td, struct tdx_module_args *ar
> {
> 	args->rcx = tdx_tdvpr_pa(td);
> 
>-	return __seamcall_saved_ret(TDH_VP_ENTER, args);
>+	return do_seamcall(__seamcall_saved_ret, TDH_VP_ENTER, args);
> }
> EXPORT_SYMBOL_GPL(tdh_vp_enter);
> 
>-- 
>2.50.0
>