Re: [PATCH 1/2] KVM: For manual-protect GET_DIRTY_LOG, do not hold slots lock

[Dan Carpenter <dan.carpenter@linaro.org>]

Hi James,

kernel test robot noticed the following build warnings:

url:    https://github.com/intel-lab-lkp/linux/commits/James-Houghton/KVM-selftests-Add-parallel-KVM_GET_DIRTY_LOG-to-dirty_log_perf_test/20251001-013306
base:   a6ad54137af92535cfe32e19e5f3bc1bb7dbd383
patch link:    https://lore.kernel.org/r/20250930172850.598938-1-jthoughton%40google.com
patch subject: [PATCH 1/2] KVM: For manual-protect GET_DIRTY_LOG, do not hold slots lock
config: x86_64-randconfig-161-20251004 (https://download.01.org/0day-ci/archive/20251004/202510041919.LaZWBcDN-lkp@intel.com/config)
compiler: clang version 20.1.8 (https://github.com/llvm/llvm-project 87f0227cb60147a26a1eeb4fb06e3b505e9c7261)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
| Closes: https://lore.kernel.org/r/202510041919.LaZWBcDN-lkp@intel.com/

New smatch warnings:
arch/x86/kvm/../../../virt/kvm/kvm_main.c:2290 kvm_get_dirty_log_protect() error: uninitialized symbol 'flush'.

vim +/flush +2290 arch/x86/kvm/../../../virt/kvm/kvm_main.c

ba0513b5b8ffbcb virt/kvm/kvm_main.c    Mario Smarduch      2015-01-15  2255  	n = kvm_dirty_bitmap_bytes(memslot);
82fb1294f7ad3ee virt/kvm/kvm_main.c    James Houghton      2025-09-30  2256  	if (!protect) {
2a31b9db153530d virt/kvm/kvm_main.c    Paolo Bonzini       2018-10-23  2257  		/*
82fb1294f7ad3ee virt/kvm/kvm_main.c    James Houghton      2025-09-30  2258  		 * Unlike kvm_get_dirty_log, we never flush, because no flush is
82fb1294f7ad3ee virt/kvm/kvm_main.c    James Houghton      2025-09-30  2259  		 * needed until KVM_CLEAR_DIRTY_LOG.  There is some code
82fb1294f7ad3ee virt/kvm/kvm_main.c    James Houghton      2025-09-30  2260  		 * duplication between this function and kvm_get_dirty_log, but
82fb1294f7ad3ee virt/kvm/kvm_main.c    James Houghton      2025-09-30  2261  		 * hopefully all architecture transition to
82fb1294f7ad3ee virt/kvm/kvm_main.c    James Houghton      2025-09-30  2262  		 * kvm_get_dirty_log_protect and kvm_get_dirty_log can be
82fb1294f7ad3ee virt/kvm/kvm_main.c    James Houghton      2025-09-30  2263  		 * eliminated.
2a31b9db153530d virt/kvm/kvm_main.c    Paolo Bonzini       2018-10-23  2264  		 */
2a31b9db153530d virt/kvm/kvm_main.c    Paolo Bonzini       2018-10-23  2265  		dirty_bitmap_buffer = dirty_bitmap;
2a31b9db153530d virt/kvm/kvm_main.c    Paolo Bonzini       2018-10-23  2266  	} else {
82fb1294f7ad3ee virt/kvm/kvm_main.c    James Houghton      2025-09-30  2267  		bool flush;

flush needs to be initialized to false.

82fb1294f7ad3ee virt/kvm/kvm_main.c    James Houghton      2025-09-30  2268  
03133347b4452ef virt/kvm/kvm_main.c    Claudio Imbrenda    2018-04-30  2269  		dirty_bitmap_buffer = kvm_second_dirty_bitmap(memslot);
ba0513b5b8ffbcb virt/kvm/kvm_main.c    Mario Smarduch      2015-01-15  2270  		memset(dirty_bitmap_buffer, 0, n);
ba0513b5b8ffbcb virt/kvm/kvm_main.c    Mario Smarduch      2015-01-15  2271  
531810caa9f4bc9 virt/kvm/kvm_main.c    Ben Gardon          2021-02-02  2272  		KVM_MMU_LOCK(kvm);
ba0513b5b8ffbcb virt/kvm/kvm_main.c    Mario Smarduch      2015-01-15  2273  		for (i = 0; i < n / sizeof(long); i++) {
ba0513b5b8ffbcb virt/kvm/kvm_main.c    Mario Smarduch      2015-01-15  2274  			unsigned long mask;
ba0513b5b8ffbcb virt/kvm/kvm_main.c    Mario Smarduch      2015-01-15  2275  			gfn_t offset;
ba0513b5b8ffbcb virt/kvm/kvm_main.c    Mario Smarduch      2015-01-15  2276  
ba0513b5b8ffbcb virt/kvm/kvm_main.c    Mario Smarduch      2015-01-15  2277  			if (!dirty_bitmap[i])
ba0513b5b8ffbcb virt/kvm/kvm_main.c    Mario Smarduch      2015-01-15  2278  				continue;
ba0513b5b8ffbcb virt/kvm/kvm_main.c    Mario Smarduch      2015-01-15  2279  
0dff084607bd555 virt/kvm/kvm_main.c    Sean Christopherson 2020-02-18  2280  			flush = true;
ba0513b5b8ffbcb virt/kvm/kvm_main.c    Mario Smarduch      2015-01-15  2281  			mask = xchg(&dirty_bitmap[i], 0);
ba0513b5b8ffbcb virt/kvm/kvm_main.c    Mario Smarduch      2015-01-15  2282  			dirty_bitmap_buffer[i] = mask;
ba0513b5b8ffbcb virt/kvm/kvm_main.c    Mario Smarduch      2015-01-15  2283  
ba0513b5b8ffbcb virt/kvm/kvm_main.c    Mario Smarduch      2015-01-15  2284  			offset = i * BITS_PER_LONG;
58d2930f4ee335a virt/kvm/kvm_main.c    Takuya Yoshikawa    2015-03-17  2285  			kvm_arch_mmu_enable_log_dirty_pt_masked(kvm, memslot,
58d2930f4ee335a virt/kvm/kvm_main.c    Takuya Yoshikawa    2015-03-17  2286  								offset, mask);
58d2930f4ee335a virt/kvm/kvm_main.c    Takuya Yoshikawa    2015-03-17  2287  		}
531810caa9f4bc9 virt/kvm/kvm_main.c    Ben Gardon          2021-02-02  2288  		KVM_MMU_UNLOCK(kvm);
2a31b9db153530d virt/kvm/kvm_main.c    Paolo Bonzini       2018-10-23  2289  
0dff084607bd555 virt/kvm/kvm_main.c    Sean Christopherson 2020-02-18 @2290  		if (flush)

Either uninitialized or true.  Never false.

619b5072443c05c virt/kvm/kvm_main.c    David Matlack       2023-08-11  2291  			kvm_flush_remote_tlbs_memslot(kvm, memslot);
82fb1294f7ad3ee virt/kvm/kvm_main.c    James Houghton      2025-09-30  2292  	}
0dff084607bd555 virt/kvm/kvm_main.c    Sean Christopherson 2020-02-18  2293  
ba0513b5b8ffbcb virt/kvm/kvm_main.c    Mario Smarduch      2015-01-15  2294  	if (copy_to_user(log->dirty_bitmap, dirty_bitmap_buffer, n))
58d6db349172786 virt/kvm/kvm_main.c    Markus Elfring      2017-01-22  2295  		return -EFAULT;
58d6db349172786 virt/kvm/kvm_main.c    Markus Elfring      2017-01-22  2296  	return 0;
ba0513b5b8ffbcb virt/kvm/kvm_main.c    Mario Smarduch      2015-01-15  2297  }

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