linux-kernel - Re: [PATCH v3 1/2] x86/sev: Put PSC struct on the stack in prep for unaccepted memory support

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Date:   Wed, 17 Aug 2022 18:08:19 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        "H. Peter Anvin" <hpa@...or.com>,
        Michael Roth <michael.roth@....com>,
        Joerg Roedel <jroedel@...e.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 1/2] x86/sev: Put PSC struct on the stack in prep for
 unaccepted memory support

On Mon, Aug 15, 2022 at 10:57:42AM -0500, Tom Lendacky wrote:
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index c05f0124c410..40268ce97aad 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -66,6 +66,17 @@ static struct ghcb boot_ghcb_page __bss_decrypted __aligned(PAGE_SIZE);
>   */
>  static struct ghcb *boot_ghcb __section(".data");
>  
> +/*
> + * A flag used by set_pages_state() that indicates when the per-CPU GHCB has
> + * been created and registered and thus can be used instead of using the MSR
> + * protocol. The set_pages_state() function eventually invokes vmgexit_psc(),
> + * which only works with a per-CPU GHCB.
> + *
> + * For APs, the per-CPU GHCB is created before they are started and registered
> + * upon startup, so this flag can be used globally for the BSP and APs.
> + */

Ok, better, thanks!

> +static bool ghcb_percpu_ready __section(".data");

However, it reads really weird if you have "percpu" in the name of a
variable which is not per CPU...

Let's just call it "ghcbs_initialized" and be done with it.

And I still hate the whole thing ofc.

Do this ontop (and I knew we had a flags thing already):

(And yes, __read_mostly is in the .data section too).

---
diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 40268ce97aad..5b3afbf26349 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -66,17 +66,6 @@ static struct ghcb boot_ghcb_page __bss_decrypted __aligned(PAGE_SIZE);
  */
 static struct ghcb *boot_ghcb __section(".data");
 
-/*
- * A flag used by set_pages_state() that indicates when the per-CPU GHCB has
- * been created and registered and thus can be used instead of using the MSR
- * protocol. The set_pages_state() function eventually invokes vmgexit_psc(),
- * which only works with a per-CPU GHCB.
- *
- * For APs, the per-CPU GHCB is created before they are started and registered
- * upon startup, so this flag can be used globally for the BSP and APs.
- */
-static bool ghcb_percpu_ready __section(".data");
-
 /* Bitmap of SEV features supported by the hypervisor */
 static u64 sev_hv_features __ro_after_init;
 
@@ -128,7 +117,18 @@ static DEFINE_PER_CPU(struct sev_es_save_area *, sev_vmsa);
 
 struct sev_config {
 	__u64 debug		: 1,
-	      __reserved	: 63;
+
+	      /*
+	       * A flag used by set_pages_state() that indicates when the per-CPU GHCB has
+	       * been created and registered and thus can be used instead of using the MSR
+	       * protocol. The set_pages_state() function eventually invokes vmgexit_psc(),
+	       * which only works with a per-CPU GHCB.
+	       *
+	       * For APs, the per-CPU GHCB is created before they are started and registered
+	       * upon startup, so this flag can be used globally for the BSP and APs.
+	       */
+	      ghcbs_initialized : 1,
+	      __reserved	: 62;
 };
 
 static struct sev_config sev_cfg __read_mostly;
@@ -762,7 +762,7 @@ static int vmgexit_psc(struct snp_psc_desc *desc)
 	unsigned long flags;
 	struct ghcb *ghcb;
 
-	WARN_ON_ONCE(!ghcb_percpu_ready);
+	WARN_ON_ONCE(!sev_cfg.ghcbs_initialized);
 
 	/*
 	 * __sev_get_ghcb() needs to run with IRQs disabled because it is using
@@ -887,7 +887,7 @@ static void set_pages_state(unsigned long vaddr, unsigned int npages, int op)
 	 * Use the MSR protocol when the per-CPU GHCBs are not yet registered,
 	 * since vmgexit_psc() uses the per-CPU GHCB.
 	 */
-	if (!ghcb_percpu_ready)
+	if (!sev_cfg.ghcbs_initialized)
 		return early_set_pages_state(__pa(vaddr), npages, op);
 
 	vaddr = vaddr & PAGE_MASK;
@@ -1268,7 +1268,7 @@ void setup_ghcb(void)
 		if (cc_platform_has(CC_ATTR_GUEST_SEV_SNP))
 			snp_register_per_cpu_ghcb();
 
-		ghcb_percpu_ready = true;
+		sev_cfg.ghcbs_initialized = true;
 
 		return;
 	}

-- 
Regards/Gruss,
    Boris.

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