Message-ID: <42917119-b43a-062b-6c09-13b988f7194b@amd.com>
Date:   Mon, 8 Mar 2021 12:52:59 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH 20/24] KVM: x86/mmu: Use a dedicated bit to track
 shadow/MMU-present SPTEs

On 2/25/21 2:47 PM, Sean Christopherson wrote:
> Introduce MMU_PRESENT to explicitly track which SPTEs are "present" from
> the MMU's perspective.  Checking for shadow-present SPTEs is a very
> common operation for the MMU, particularly in hot paths such as page
> faults.  With the addition of "removed" SPTEs for the TDP MMU,
> identifying shadow-present SPTEs is quite costly especially since it
> requires checking multiple 64-bit values.
> 
> On 64-bit KVM, this reduces the footprint of kvm.ko's .text by ~2k bytes.
> On 32-bit KVM, this increases the footprint by ~200 bytes, but only
> because gcc now inlines several more MMU helpers, e.g. drop_parent_pte().
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/mmu/spte.c |  8 ++++----
>   arch/x86/kvm/mmu/spte.h | 11 ++++++++++-
>   2 files changed, 14 insertions(+), 5 deletions(-)

I'm trying to run a guest on my Rome system using the queue branch, but
I'm encountering an error that I bisected to this commit. In the guest
(during OVMF boot) I see:

error: kvm run failed Invalid argument
RAX=0000000000000000 RBX=00000000ffc12792 RCX=000000007f58401a RDX=000000007faaf808
RSI=0000000000000010 RDI=00000000ffc12792 RBP=00000000ffc12792 RSP=000000007faaf740
R8 =0000000000000792 R9 =000000007faaf808 R10=00000000ffc12793 R11=00000000000003f8
R12=0000000000000010 R13=0000000000000000 R14=000000007faaf808 R15=0000000000000012
RIP=000000007f6e9a90 RFL=00000246 [---Z-P-] CPL=0 II=0 A20=1 SMM=0 HLT=0
ES =0030 0000000000000000 ffffffff 00c09300 DPL=0 DS   [-WA]
CS =0038 0000000000000000 ffffffff 00a09b00 DPL=0 CS64 [-RA]
SS =0030 0000000000000000 ffffffff 00c09300 DPL=0 DS   [-WA]
DS =0030 0000000000000000 ffffffff 00c09300 DPL=0 DS   [-WA]
FS =0030 0000000000000000 ffffffff 00c09300 DPL=0 DS   [-WA]
GS =0030 0000000000000000 ffffffff 00c09300 DPL=0 DS   [-WA]
LDT=0000 0000000000000000 0000ffff 00008200 DPL=0 LDT
TR =0000 0000000000000000 0000ffff 00008b00 DPL=0 TSS64-busy
GDT=     000000007f5ee698 00000047
IDT=     000000007f186018 00000fff
CR0=80010033 CR2=0000000000000000 CR3=000000007f801000 CR4=00000668
DR0=0000000000000000 DR1=0000000000000000 DR2=0000000000000000 DR3=0000000000000000 
DR6=00000000ffff0ff0 DR7=0000000000000400
EFER=0000000000000d00
Code=22 00 00 e8 c0 e6 ff ff 48 83 c4 20 45 84 ed 74 07 fb eb 04 <44> 88 65 00 58 5b 5d 41 5c 41 5d c3 55 48 0f af 3d 1b 37 00 00 be 20 00 00 00 48 03 3d 17

On the hypervisor, I see the following:

[   55.886136] get_mmio_spte: detect reserved bits on spte, addr 0xffc12792, dump hierarchy:
[   55.895284] ------ spte 0x1344a0827 level 4.
[   55.900059] ------ spte 0x134499827 level 3.
[   55.904877] ------ spte 0x165bf0827 level 2.
[   55.909651] ------ spte 0xff800ffc12817 level 1.

When I kill the guest, I get a kernel panic:

[   95.539683] __pte_list_remove: 0000000040567a6a 0->BUG
[   95.545481] kernel BUG at arch/x86/kvm/mmu/mmu.c:896!
[   95.551133] invalid opcode: 0000 [#1] SMP NOPTI
[   95.556192] CPU: 142 PID: 5054 Comm: qemu-system-x86 Tainted: G        W         5.11.0-rc4-sos-sev-es #1
[   95.566872] Hardware name: AMD Corporation ETHANOL_X/ETHANOL_X, BIOS REX1006G 01/25/2020
[   95.575900] RIP: 0010:__pte_list_remove.cold+0x2e/0x48 [kvm]
[   95.582312] Code: c7 c6 40 6f f3 c0 48 c7 c7 aa da f3 c0 e8 79 3d a7 cd 0f 0b 48 89 fa 48 c7 c6 40 6f f3 c0 48 c7 c7 87 da f3 c0 e8 61 3d a7 cd <0f> 0b 48 89 fa 48 c7 c6 40 6f f3 c0 48 c7 c7 98 da f3 c0 e8 49 3d
[   95.603271] RSP: 0018:ffffc900143e7c78 EFLAGS: 00010246
[   95.609093] RAX: 000000000000002a RBX: 0000000000000000 RCX: 0000000000000000
[   95.617058] RDX: 0000000000000000 RSI: ffff88900e598950 RDI: ffff88900e598950
[   95.625019] RBP: ffff888165bf0090 R08: ffff88900e598950 R09: ffffc900143e7a98
[   95.632980] R10: 0000000000000001 R11: 0000000000000001 R12: ffffc9000ff29000
[   95.640944] R13: ffffc900143e7d18 R14: 0000000000000098 R15: 0000000000000000
[   95.648912] FS:  0000000000000000(0000) GS:ffff88900e580000(0000) knlGS:0000000000000000
[   95.657951] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   95.664361] CR2: 00007fb328d20c80 CR3: 00000001476d2000 CR4: 0000000000350ee0
[   95.672326] Call Trace:
[   95.675065]  mmu_page_zap_pte+0xf9/0x130 [kvm]
[   95.680103]  __kvm_mmu_prepare_zap_page+0x6d/0x380 [kvm]
[   95.686088]  kvm_mmu_zap_all+0x5e/0xe0 [kvm]
[   95.690911]  kvm_mmu_notifier_release+0x2b/0x60 [kvm]
[   95.696614]  __mmu_notifier_release+0x71/0x1e0
[   95.701585]  ? asm_sysvec_apic_timer_interrupt+0x12/0x20
[   95.707512]  ? __khugepaged_exit+0x111/0x160
[   95.712289]  exit_mmap+0x15b/0x1f0
[   95.716092]  ? __khugepaged_exit+0x111/0x160
[   95.720857]  ? kmem_cache_free+0x210/0x3f0
[   95.725428]  ? kmem_cache_free+0x387/0x3f0
[   95.729998]  mmput+0x56/0x130
[   95.733312]  do_exit+0x341/0xb50
[   95.736923]  do_group_exit+0x3a/0xa0
[   95.740925]  __x64_sys_exit_group+0x14/0x20
[   95.745600]  do_syscall_64+0x33/0x40
[   95.749601]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[   95.755241] RIP: 0033:0x7fb333a442c6
[   95.759231] Code: Unable to access opcode bytes at RIP 0x7fb333a4429c.
[   95.766514] RSP: 002b:00007ffdf675cad8 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
[   95.774962] RAX: ffffffffffffffda RBX: 00007fb333b4b610 RCX: 00007fb333a442c6
[   95.782925] RDX: 0000000000000000 RSI: 000000000000003c RDI: 0000000000000000
[   95.790892] RBP: 0000000000000000 R08: 00000000000000e7 R09: ffffffffffffdc38
[   95.798856] R10: 00007fb32945cf80 R11: 0000000000000246 R12: 00007fb333b4b610
[   95.806825] R13: 000000000000034c R14: 00007fb333b4efc8 R15: 0000000000000000
[   95.814803] Modules linked in: tun ebtable_filter ebtables ip6table_filter ip6_tables iptable_filter bridge stp llc intel_rapl_msr wmi_bmof intel_rapl_common amd64_edac_mod edac_mce_amd fuse kvm_amd kvm irqby
pass ipmi_ssif sg ccp k10temp acpi_ipmi ipmi_si ipmi_devintf ipmi_msghandler acpi_cpufreq sch_fq_codel parport_pc ppdev lp parport sunrpc ip_tables raid10 raid456 async_raid6_recov async_memcpy async_pq async_xo
r async_tx xor raid6_pq raid1 raid0 linear sd_mod t10_pi crct10dif_pclmul crc32_pclmul crc32c_intel ghash_clmulni_intel aesni_intel crypto_simd cryptd glue_helper ast drm_vram_helper i2c_algo_bit drm_ttm_helper 
ttm drm_kms_helper syscopyarea sysfillrect sysimgblt ahci fb_sys_fops libahci libata drm i2c_designware_platform e1000e i2c_piix4 wmi i2c_designware_core pinctrl_amd i2c_core
[   95.893646] ---[ end trace f40aac7ee7919c14 ]---
[   95.898848] RIP: 0010:__pte_list_remove.cold+0x2e/0x48 [kvm]
[   95.905258] Code: c7 c6 40 6f f3 c0 48 c7 c7 aa da f3 c0 e8 79 3d a7 cd 0f 0b 48 89 fa 48 c7 c6 40 6f f3 c0 48 c7 c7 87 da f3 c0 e8 61 3d a7 cd <0f> 0b 48 89 fa 48 c7 c6 40 6f f3 c0 48 c7 c7 98 da f3 c0 e8 49 3d
[   95.926234] RSP: 0018:ffffc900143e7c78 EFLAGS: 00010246
[   95.932109] RAX: 000000000000002a RBX: 0000000000000000 RCX: 0000000000000000
[   95.940087] RDX: 0000000000000000 RSI: ffff88900e598950 RDI: ffff88900e598950
[   95.948086] RBP: ffff888165bf0090 R08: ffff88900e598950 R09: ffffc900143e7a98
[   95.956068] R10: 0000000000000001 R11: 0000000000000001 R12: ffffc9000ff29000
[   95.964051] R13: ffffc900143e7d18 R14: 0000000000000098 R15: 0000000000000000
[   95.972031] FS:  0000000000000000(0000) GS:ffff88900e580000(0000) knlGS:0000000000000000
[   95.981081] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   95.987510] CR2: 00007fb328d20c80 CR3: 00000001476d2000 CR4: 0000000000350ee0
[   95.995492] Kernel panic - not syncing: Fatal exception
[   96.008273] Kernel Offset: disabled
[   96.012249] ---[ end Kernel panic - not syncing: Fatal exception ]---

Let me know if there's anything you want me to try.

Thanks,
Tom

> 
> diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
> index d12acf5eb871..e07aabb23b8a 100644
> --- a/arch/x86/kvm/mmu/spte.c
> +++ b/arch/x86/kvm/mmu/spte.c
> @@ -94,7 +94,7 @@ int make_spte(struct kvm_vcpu *vcpu, unsigned int pte_access, int level,
>   		     bool can_unsync, bool host_writable, bool ad_disabled,
>   		     u64 *new_spte)
>   {
> -	u64 spte = 0;
> +	u64 spte = SPTE_MMU_PRESENT_MASK;
>   	int ret = 0;
>   
>   	if (ad_disabled)
> @@ -183,10 +183,10 @@ int make_spte(struct kvm_vcpu *vcpu, unsigned int pte_access, int level,
>   
>   u64 make_nonleaf_spte(u64 *child_pt, bool ad_disabled)
>   {
> -	u64 spte;
> +	u64 spte = SPTE_MMU_PRESENT_MASK;
>   
> -	spte = __pa(child_pt) | shadow_present_mask | PT_WRITABLE_MASK |
> -	       shadow_user_mask | shadow_x_mask | shadow_me_mask;
> +	spte |= __pa(child_pt) | shadow_present_mask | PT_WRITABLE_MASK |
> +		shadow_user_mask | shadow_x_mask | shadow_me_mask;
>   
>   	if (ad_disabled)
>   		spte |= SPTE_TDP_AD_DISABLED_MASK;
> diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
> index 8996baa8da15..645e9bc2d4a2 100644
> --- a/arch/x86/kvm/mmu/spte.h
> +++ b/arch/x86/kvm/mmu/spte.h
> @@ -5,6 +5,15 @@
>   
>   #include "mmu_internal.h"
>   
> +/*
> + * A MMU present SPTE is backed by actual memory and may or may not be present
> + * in hardware.  E.g. MMIO SPTEs are not considered present.  Use bit 11, as it
> + * is ignored by all flavors of SPTEs and checking a low bit often generates
> + * better code than for a high bit, e.g. 56+.  MMU present checks are pervasive
> + * enough that the improved code generation is noticeable in KVM's footprint.
> + */
> +#define SPTE_MMU_PRESENT_MASK		BIT_ULL(11)
> +
>   /*
>    * TDP SPTES (more specifically, EPT SPTEs) may not have A/D bits, and may also
>    * be restricted to using write-protection (for L2 when CPU dirty logging, i.e.
> @@ -241,7 +250,7 @@ static inline bool is_access_track_spte(u64 spte)
>   
>   static inline bool is_shadow_present_pte(u64 pte)
>   {
> -	return (pte != 0) && !is_mmio_spte(pte) && !is_removed_spte(pte);
> +	return !!(pte & SPTE_MMU_PRESENT_MASK);
>   }
>   
>   static inline bool is_large_pte(u64 pte)
> 

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