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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Message-ID: <97badacb-7477-5a85-d089-4a5156b57e30@amd.com>
Date:   Wed, 17 Aug 2022 16:17:54 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Borislav Petkov <bp@...en8.de>
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 8/17/22 11:08, Borislav Petkov wrote:
> 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).

Cool, will do.

Thanks,
Tom

> 
> ---
> 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;
>   	}
>