Re: [PATCH 1/2] Revert "KVM: x86: Truncate reported guest MAXPHYADDR to C-bit if SEV is supported"

[Tom Lendacky]

On 6/24/21 9:03 PM, Sean Christopherson wrote:
> It turns out that non-SEV guest can indeed use bit 47, and the unexpected
> reserved #PF observed when setting bit 47 is due to a magic 12gb
> HyperTransport region being off limits, even for GPAs.  Per Tom:
> 
>   I think you may be hitting a special HT region that is at the top 12GB
>   of the 48-bit memory range and is reserved, even for GPAs.  Can you
>   somehow get the test to use an address below 0xfffd_0000_0000? That
>   would show that bit 47 is valid for the legacy guest while staying out
>   of the HT region.
> 
> And indeed, accessing 0xfffd00000000 generates a reserved #PF, while
> dropping down a single page to 0xfffcfffff000 does not.
> 
> This reverts commit 3675f005c87c4026713c9f863924de511fdd36c4.
> 
> Cc: Peter Gonda <pgonda@xxxxxxxxxx>
> Cc: Brijesh Singh <brijesh.singh@xxxxxxx>
> Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Acked-by: Tom Lendacky <thomas.lendacky@xxxxxxx>

> ---
>  arch/x86/kvm/cpuid.c   | 11 -----------
>  arch/x86/kvm/svm/svm.c | 37 ++++++++-----------------------------
>  arch/x86/kvm/x86.c     |  3 ---
>  arch/x86/kvm/x86.h     |  1 -
>  4 files changed, 8 insertions(+), 44 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 0edda1fc4fe7..ca7866d63e98 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -955,17 +955,6 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>  		else if (!g_phys_as)
>  			g_phys_as = phys_as;
>  
> -		/*
> -		 * The exception to the exception is if hardware supports SEV,
> -		 * in which case the C-bit is reserved for non-SEV guests and
> -		 * isn't a GPA bit for SEV guests.
> -		 *
> -		 * Note, KVM always reports '0' for the number of reduced PA
> -		 * bits (see 0x8000001F).
> -		 */
> -		if (tdp_enabled && sev_c_bit)
> -			g_phys_as = min(g_phys_as, (unsigned int)sev_c_bit);
> -
>  		entry->eax = g_phys_as | (virt_as << 8);
>  		entry->edx = 0;
>  		cpuid_entry_override(entry, CPUID_8000_0008_EBX);
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 616b9679ddcc..8834822c00cd 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -860,26 +860,6 @@ static void shrink_ple_window(struct kvm_vcpu *vcpu)
>  	}
>  }
>  
> -static __init u8 svm_get_c_bit(bool sev_only)
> -{
> -	unsigned int eax, ebx, ecx, edx;
> -	u64 msr;
> -
> -	if (cpuid_eax(0x80000000) < 0x8000001f)
> -		return 0;
> -
> -	if (rdmsrl_safe(MSR_AMD64_SYSCFG, &msr) ||
> -	    !(msr & MSR_AMD64_SYSCFG_MEM_ENCRYPT))
> -		return 0;
> -
> -	cpuid(0x8000001f, &eax, &ebx, &ecx, &edx);
> -
> -	if (sev_only && !(eax & feature_bit(SEV)))
> -		return 0;
> -
> -	return ebx & 0x3f;
> -}
> -
>  /*
>   * The default MMIO mask is a single bit (excluding the present bit),
>   * which could conflict with the memory encryption bit. Check for
> @@ -889,13 +869,18 @@ static __init u8 svm_get_c_bit(bool sev_only)
>  static __init void svm_adjust_mmio_mask(void)
>  {
>  	unsigned int enc_bit, mask_bit;
> -	u64 mask;
> +	u64 msr, mask;
> +
> +	/* If there is no memory encryption support, use existing mask */
> +	if (cpuid_eax(0x80000000) < 0x8000001f)
> +		return;
>  
>  	/* If memory encryption is not enabled, use existing mask */
> -	enc_bit = svm_get_c_bit(false);
> -	if (!enc_bit)
> +	rdmsrl(MSR_AMD64_SYSCFG, msr);
> +	if (!(msr & MSR_AMD64_SYSCFG_MEM_ENCRYPT))
>  		return;
>  
> +	enc_bit = cpuid_ebx(0x8000001f) & 0x3f;
>  	mask_bit = boot_cpu_data.x86_phys_bits;
>  
>  	/* Increment the mask bit if it is the same as the encryption bit */
> @@ -1028,12 +1013,6 @@ static __init int svm_hardware_setup(void)
>  	kvm_configure_mmu(npt_enabled, get_max_npt_level(), PG_LEVEL_1G);
>  	pr_info("kvm: Nested Paging %sabled\n", npt_enabled ? "en" : "dis");
>  
> -	/*
> -	 * The SEV C-bit location is needed to correctly enumeration guest
> -	 * MAXPHYADDR even if SEV is not fully supported.
> -	 */
> -	sev_c_bit = svm_get_c_bit(true);
> -
>  	/* Note, SEV setup consumes npt_enabled. */
>  	sev_hardware_setup();
>  
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 4a597aafe637..13905ef5bb48 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -209,9 +209,6 @@ static struct kvm_user_return_msrs __percpu *user_return_msrs;
>  				| XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
>  				| XFEATURE_MASK_PKRU)
>  
> -u8 __read_mostly sev_c_bit;
> -EXPORT_SYMBOL_GPL(sev_c_bit);
> -
>  u64 __read_mostly host_efer;
>  EXPORT_SYMBOL_GPL(host_efer);
>  
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index bc3f5c9e3708..44ae10312740 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -328,7 +328,6 @@ extern u64 host_xcr0;
>  extern u64 supported_xcr0;
>  extern u64 host_xss;
>  extern u64 supported_xss;
> -extern u8  sev_c_bit;
>  
>  static inline bool kvm_mpx_supported(void)
>  {
>